port async unlinked drain from illumos-nexenta

This patch is an async implementation of the existing sync
zfs_unlinked_drain() function. This function is called at mount time and
is responsible for freeing znodes that we didn't get to freeing before.
We don't have to hold mounting of the dataset until the unlinked list is
fully drained as is done now. Since we can process the unlinked set
asynchronously this results in a better user experience when mounting a
dataset with entries in the unlinked set.

Reviewed by: Jorgen Lundman <lundman@lundman.net>
Reviewed by: Tom Caputi <tcaputi@datto.com>
Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Matt Ahrens <mahrens@delphix.com>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Signed-off-by: Alek Pinchuk <apinchuk@datto.com>
Closes #8142

include/sys/zfs_vfsops.h

@@ -117,6 +117,8 @@ struct zfsvfs {
 	boolean_t	z_replay;	/* set during ZIL replay */
 	boolean_t	z_use_sa;	/* version allow system attributes */
 	boolean_t	z_xattr_sa;	/* allow xattrs to be stores as SA */
+	boolean_t	z_draining;	/* is true when drain is active */
+	boolean_t	z_drain_cancel; /* signal the unlinked drain to stop */
 	uint64_t	z_version;	/* ZPL version */
 	uint64_t	z_shares_dir;	/* hidden shares dir */
 	dataset_kstats_t	z_kstat;	/* fs kstats */
@@ -132,6 +134,7 @@ struct zfsvfs {
 	uint64_t	z_hold_size;	/* znode hold array size */
 	avl_tree_t	*z_hold_trees;	/* znode hold trees */
 	kmutex_t	*z_hold_locks;	/* znode hold locks */
+	taskqid_t	z_drain_task;	/* task id for the unlink drain task */
 };
 
 #define	ZSB_XATTR	0x0001		/* Enable user xattrs */