xfs: fix stray dquot unlock when reclaiming dquots

When we fail to get a dquot lock during reclaim, we jump to an error
handler that unlocks the dquot. This is wrong as we didn't lock the
dquot, and unlocking it means who-ever is holding the lock has had
it silently taken away, and hence it results in a lock imbalance.

Found by inspection while modifying the code for the numa-lru
patchset. This fixes a random hang I've been seeing on xfstest 232
for the past several months.

cc: <stable@vger.kernel.org>
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ben Myers <bpm@sgi.com>

fs/xfs/xfs_qm.c

@@ -1456,7 +1456,7 @@ xfs_qm_dqreclaim_one(
 	int			error;
 
 	if (!xfs_dqlock_nowait(dqp))
-		goto out_busy;
+		goto out_move_tail;
 
 	/*
 	 * This dquot has acquired a reference in the meantime remove it from
@@ -1479,7 +1479,7 @@ xfs_qm_dqreclaim_one(
 	 * getting flushed to disk, we don't want to reclaim it.
 	 */
 	if (!xfs_dqflock_nowait(dqp))
-		goto out_busy;
+		goto out_unlock_move_tail;
 
 	if (XFS_DQ_IS_DIRTY(dqp)) {
 		struct xfs_buf	*bp = NULL;
@@ -1490,7 +1490,7 @@ xfs_qm_dqreclaim_one(
 		if (error) {
 			xfs_warn(mp, "%s: dquot %p flush failed",
 				 __func__, dqp);
-			goto out_busy;
+			goto out_unlock_move_tail;
 		}
 
 		xfs_buf_delwri_queue(bp, buffer_list);
@@ -1499,7 +1499,7 @@ xfs_qm_dqreclaim_one(
 		 * Give the dquot another try on the freelist, as the
 		 * flushing will take some time.
 		 */
-		goto out_busy;
+		goto out_unlock_move_tail;
 	}
 	xfs_dqfunlock(dqp);
 
@@ -1518,14 +1518,13 @@ xfs_qm_dqreclaim_one(
 	XFS_STATS_INC(xs_qm_dqreclaims);
 	return;
 
-out_busy:
-	xfs_dqunlock(dqp);
-
 	/*
 	 * Move the dquot to the tail of the list so that we don't spin on it.
 	 */
+out_unlock_move_tail:
+	xfs_dqunlock(dqp);
+out_move_tail:
 	list_move_tail(&dqp->q_lru, &qi->qi_lru_list);
-
 	trace_xfs_dqreclaim_busy(dqp);
 	XFS_STATS_INC(xs_qm_dqreclaim_misses);
 }