Throw const on some strings

In C, const indicates to the reader that mutation will not occur.
It can also serve as a hint about ownership.

Add const in a few places where it makes sense.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <freqlabs@FreeBSD.org>
Closes #10997

include/os/freebsd/zfs/sys/zfs_ctldir.h

@@ -48,7 +48,7 @@ int zfsctl_root(zfsvfs_t *, int, vnode_t **);
 void zfsctl_init(void);
 void zfsctl_fini(void);
 boolean_t zfsctl_is_node(vnode_t *);
-int zfsctl_snapshot_unmount(char *snapname, int flags);
+int zfsctl_snapshot_unmount(const char *snapname, int flags);
 int zfsctl_rename_snapshot(const char *from, const char *to);
 int zfsctl_destroy_snapshot(const char *snapname, int force);
 int zfsctl_umount_snapshots(vfs_t *, int, cred_t *);

include/os/freebsd/zfs/sys/zfs_dir.h

@@ -52,11 +52,7 @@ extern int zfs_dirent_lookup(znode_t *, const char *, znode_t **, int);
 extern int zfs_link_create(znode_t *, const char *, znode_t *, dmu_tx_t *, int);
 extern int zfs_link_destroy(znode_t *, const char *, znode_t *, dmu_tx_t *, int,
     boolean_t *);
-#if 0
-extern int zfs_dirlook(vnode_t *, const char *, vnode_t **, int);
-#else
 extern int zfs_dirlook(znode_t *, const char *name, znode_t **);
-#endif
 extern void zfs_mknode(znode_t *, vattr_t *, dmu_tx_t *, cred_t *,
     uint_t, znode_t **, zfs_acl_ids_t *);
 extern void zfs_rmnode(znode_t *);

include/os/freebsd/zfs/sys/zfs_vnops.h

@@ -32,21 +32,21 @@ int dmu_write_pages(objset_t *os, uint64_t object, uint64_t offset,
     uint64_t size, struct vm_page **ppa, dmu_tx_t *tx);
 int dmu_read_pages(objset_t *os, uint64_t object, vm_page_t *ma, int count,
     int *rbehind, int *rahead, int last_size);
-extern int zfs_remove(znode_t *dzp, char *name, cred_t *cr, int flags);
-extern int zfs_mkdir(znode_t *dzp, char *dirname, vattr_t *vap,
+extern int zfs_remove(znode_t *dzp, const char *name, cred_t *cr, int flags);
+extern int zfs_mkdir(znode_t *dzp, const char *dirname, vattr_t *vap,
     znode_t **zpp, cred_t *cr, int flags, vsecattr_t *vsecp);
-extern int zfs_rmdir(znode_t *dzp, char *name, znode_t *cwd,
+extern int zfs_rmdir(znode_t *dzp, const char *name, znode_t *cwd,
     cred_t *cr, int flags);
 extern int zfs_setattr(znode_t *zp, vattr_t *vap, int flag, cred_t *cr);
-extern int zfs_rename(znode_t *sdzp, char *snm, znode_t *tdzp,
-    char *tnm, cred_t *cr, int flags);
+extern int zfs_rename(znode_t *sdzp, const char *snm, znode_t *tdzp,
+    const char *tnm, cred_t *cr, int flags);
 extern int zfs_symlink(znode_t *dzp, const char *name, vattr_t *vap,
     const char *link, znode_t **zpp, cred_t *cr, int flags);
 extern int zfs_link(znode_t *tdzp, znode_t *sp,
-    char *name, cred_t *cr, int flags);
+    const char *name, cred_t *cr, int flags);
 extern int zfs_space(znode_t *zp, int cmd, struct flock *bfp, int flag,
     offset_t offset, cred_t *cr);
-extern int zfs_create(znode_t *dzp, char *name, vattr_t *vap, int excl,
+extern int zfs_create(znode_t *dzp, const char *name, vattr_t *vap, int excl,
     int mode, znode_t **zpp, cred_t *cr, int flag, vsecattr_t *vsecp);
 extern int zfs_setsecattr(znode_t *zp, vsecattr_t *vsecp, int flag,
     cred_t *cr);

include/os/linux/zfs/sys/zfs_ctldir.h

@@ -60,22 +60,22 @@ extern boolean_t zfsctl_is_snapdir(struct inode *ip);
 extern int zfsctl_fid(struct inode *ip, fid_t *fidp);
 
 /* zfsctl '.zfs' functions */
-extern int zfsctl_root_lookup(struct inode *dip, char *name,
+extern int zfsctl_root_lookup(struct inode *dip, const char *name,
     struct inode **ipp, int flags, cred_t *cr, int *direntflags,
     pathname_t *realpnp);
 
 /* zfsctl '.zfs/snapshot' functions */
-extern int zfsctl_snapdir_lookup(struct inode *dip, char *name,
+extern int zfsctl_snapdir_lookup(struct inode *dip, const char *name,
     struct inode **ipp, int flags, cred_t *cr, int *direntflags,
     pathname_t *realpnp);
-extern int zfsctl_snapdir_rename(struct inode *sdip, char *sname,
-    struct inode *tdip, char *tname, cred_t *cr, int flags);
-extern int zfsctl_snapdir_remove(struct inode *dip, char *name, cred_t *cr,
-    int flags);
-extern int zfsctl_snapdir_mkdir(struct inode *dip, char *dirname, vattr_t *vap,
-    struct inode **ipp, cred_t *cr, int flags);
+extern int zfsctl_snapdir_rename(struct inode *sdip, const char *sname,
+    struct inode *tdip, const char *tname, cred_t *cr, int flags);
+extern int zfsctl_snapdir_remove(struct inode *dip, const char *name,
+    cred_t *cr, int flags);
+extern int zfsctl_snapdir_mkdir(struct inode *dip, const char *dirname,
+    vattr_t *vap, struct inode **ipp, cred_t *cr, int flags);
 extern int zfsctl_snapshot_mount(struct path *path, int flags);
-extern int zfsctl_snapshot_unmount(char *snapname, int flags);
+extern int zfsctl_snapshot_unmount(const char *snapname, int flags);
 extern int zfsctl_snapshot_unmount_delay(spa_t *spa, uint64_t objsetid,
     int delay);
 extern int zfsctl_snapdir_vget(struct super_block *sb, uint64_t objsetid,

include/os/linux/zfs/sys/zfs_vnops.h

@@ -46,8 +46,8 @@ extern int zfs_write(struct inode *ip, uio_t *uio, int ioflag, cred_t *cr);
 extern int zfs_write_simple(znode_t *zp, const void *data, size_t len,
     loff_t pos, size_t *resid);
 extern int zfs_access(struct inode *ip, int mode, int flag, cred_t *cr);
-extern int zfs_lookup(znode_t *dzp, char *nm, znode_t **zpp,
-    int flags, cred_t *cr, int *direntflags, pathname_t *realpnp);
+extern int zfs_lookup(znode_t *dzp, char *nm, znode_t **zpp, int flags,
+    cred_t *cr, int *direntflags, pathname_t *realpnp);
 extern int zfs_create(znode_t *dzp, char *name, vattr_t *vap, int excl,
     int mode, znode_t **zpp, cred_t *cr, int flag, vsecattr_t *vsecp);
 extern int zfs_tmpfile(struct inode *dip, vattr_t *vapzfs, int excl,