Break out of zfs_zget early if unlinked znode

If zp->z_unlinked is set, we're working with a znode that has been
marked for deletion. If that's the case, we can skip the "goto again"
loop and return ENOENT, as the znode should not be discovered.

Reviewed-by: Richard Yao <ryao@gentoo.org>
Reviewed-by: Matt Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Heitor Alves de Siqueira <halves@canonical.com>
Closes openzfs#9583

module/zfs/zfs_znode.c

@@ -1093,6 +1093,9 @@ zfs_zget(zfsvfs_t *zfsvfs, uint64_t obj_num, znode_t **zpp)
 		mutex_enter(&zp->z_lock);
 		ASSERT3U(zp->z_id, ==, obj_num);
 		/*
+		 * If zp->z_unlinked is set, the znode is already marked
+		 * for deletion and should not be discovered.
+		 *
 		 * If igrab() returns NULL the VFS has independently
 		 * determined the inode should be evicted and has
 		 * called iput_final() to start the eviction process.
@@ -1106,19 +1109,24 @@ zfs_zget(zfsvfs_t *zfsvfs, uint64_t obj_num, znode_t **zpp)
 		 * need to detect the active SA hold thereby informing
 		 * the VFS that this inode should not be evicted.
 		 */
-		if (igrab(ZTOI(zp)) == NULL) {
-			mutex_exit(&zp->z_lock);
-			sa_buf_rele(db, NULL);
-			zfs_znode_hold_exit(zfsvfs, zh);
-			/* inode might need this to finish evict */
-			cond_resched();
-			goto again;
+		if (zp->z_unlinked) {
+			err = SET_ERROR(ENOENT);
+		} else if (igrab(ZTOI(zp)) == NULL) {
+			err = SET_ERROR(EAGAIN);
+		} else {
+			*zpp = zp;
+			err = 0;
 		}
-		*zpp = zp;
-		err = 0;
+
 		mutex_exit(&zp->z_lock);
 		sa_buf_rele(db, NULL);
 		zfs_znode_hold_exit(zfsvfs, zh);
+
+		if (err == EAGAIN) {
+			/* inode might need this to finish evict */
+			cond_resched();
+			goto again;
+		}
 		return (err);
 	}