mount: use the mount syscall directly

Allow zfs datasets to be mounted on Linux without relying on the
invocation of an external processes.  This is the same behavior
which is implemented for FreeBSD.

Use of the libmount library was originally considered because it 
provides functionality to properly lock and update the /etc/mtab 
file.  However, these days /etc/mtab is typically a symlink to 
/proc/self/mounts so there's nothing to updated.  Therefore, we
call mount(2) directly and avoid any additional dependencies. 

If required the legacy behavior can be enabled by setting the 
ZFS_MOUNT_HELPER environment variable.  This may be needed in
environments where SELinux in enabled and the zfs binary does  
not have mount permission.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Felix Dörre <felix@dogcraft.de>
#10294

include/libzfs.h

@@ -803,6 +803,13 @@ extern int zfs_mount_at(zfs_handle_t *, const char *, int, const char *);
 extern int zfs_unmount(zfs_handle_t *, const char *, int);
 extern int zfs_unmountall(zfs_handle_t *, int);
 
+#if defined(__linux__)
+extern int zfs_parse_mount_options(char *mntopts, unsigned long *mntflags,
+    unsigned long *zfsflags, int sloppy, char *badopt, char *mtabopt);
+extern void zfs_adjust_mount_options(zfs_handle_t *zhp, const char *mntpoint,
+    char *mntopts, char *mtabopt);
+#endif
+
 /*
  * Share support functions.
  */

include/libzfs_impl.h

@@ -246,7 +246,8 @@ typedef struct differ_info {
 
 extern proto_table_t proto_table[PROTO_END];
 
-extern int do_mount(const char *src, const char *mntpt, char *opts, int flags);
+extern int do_mount(zfs_handle_t *zhp, const char *mntpt, char *opts,
+    int flags);
 extern int do_unmount(const char *mntpt, int flags);
 extern int zfs_mount_delegation_check(void);
 extern int zfs_share_proto(zfs_handle_t *zhp, zfs_share_proto_t *proto);