Add support for O_TMPFILE

Linux 3.11 add O_TMPFILE to open(2), which allow creating an unlinked file on
supported filesystem. It's basically doing open(2) and unlink(2) atomically.

The filesystem support is added through i_op->tmpfile. We basically copy the
create operation except we get rid of the link and name related stuff and add
the new node to unlinked set.

We also add support for linkat(2) to link tmpfile. However, since all previous
file operation will skip ZIL, we force a txg_wait_synced to make sure we are
sync safe.

Signed-off-by: Chunwei Chen <david.chen@osnexus.com>

include/sys/zfs_dir.h

@@ -47,6 +47,7 @@ extern "C" {
 /* mknode flags */
 #define	IS_ROOT_NODE	0x01		/* create a root node */
 #define	IS_XATTR	0x02		/* create an extended attribute node */
+#define	IS_TMPFILE	0x04		/* create a tmpfile */
 
 extern int zfs_dirent_lock(zfs_dirlock_t **, znode_t *, char *, znode_t **,
     int, int *, pathname_t *);

include/sys/zfs_vnops.h

@@ -47,6 +47,8 @@ extern int zfs_lookup(struct inode *dip, char *nm, struct inode **ipp,
     int flags, cred_t *cr, int *direntflags, pathname_t *realpnp);
 extern int zfs_create(struct inode *dip, char *name, vattr_t *vap, int excl,
     int mode, struct inode **ipp, cred_t *cr, int flag, vsecattr_t *vsecp);
+extern int zfs_tmpfile(struct inode *dip, vattr_t *vap, int excl,
+    int mode, struct inode **ipp, cred_t *cr, int flag, vsecattr_t *vsecp);
 extern int zfs_remove(struct inode *dip, char *name, cred_t *cr, int flags);
 extern int zfs_mkdir(struct inode *dip, char *dirname, vattr_t *vap,
     struct inode **ipp, cred_t *cr, int flags, vsecattr_t *vsecp);