xattr dir doesn't get purged during iput

We need to set inode->i_nlink to zero so iput will purge it. Without this, it
will get purged during shrink cache or umount, which would likely result in
deadlock due to zfs_zget waiting forever on its children which are in the
dispose_list of the same thread.

Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Chris Dunlop <chris@onthe.net.au>
Issue openzfs#4359
Issue openzfs#3508
Issue openzfs#4413
Issue openzfs#4827

module/zfs/zfs_dir.c

@@ -593,7 +593,7 @@ zfs_purgedir(znode_t *dzp)
 		if (error)
 			skipped += 1;
 		dmu_tx_commit(tx);
-
+		set_nlink(ZTOI(xzp), xzp->z_links);
 		zfs_iput_async(ZTOI(xzp));
 	}
 	zap_cursor_fini(&zc);
@@ -694,6 +694,7 @@ zfs_rmnode(znode_t *zp)
 		mutex_enter(&xzp->z_lock);
 		xzp->z_unlinked = B_TRUE;	/* mark xzp for deletion */
 		xzp->z_links = 0;	/* no more links to it */
+		set_nlink(ZTOI(xzp), 0); /* this will let iput purge us */
 		VERIFY(0 == sa_update(xzp->z_sa_hdl, SA_ZPL_LINKS(zsb),
 		    &xzp->z_links, sizeof (xzp->z_links), tx));
 		mutex_exit(&xzp->z_lock);