dm thin: fix issue_discard to pass GFP_NOIO to __blkdev_issue_discard

issue_discard() passes GFP_NOWAIT to __blkdev_issue_discard() despite
its code assuming bio_alloc() always succeeds.

Commit 3dba53a ("dm thin: use __blkdev_issue_discard for async
discard support") clearly shows where things went bad:

Before commit 3dba53a, dm-thin.c's open-coded
__blkdev_issue_discard_async() properly handled using GFP_NOWAIT.
Unfortunately __blkdev_issue_discard() doesn't and it was missed
during review.

Cc: stable@vger.kernel.org
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

Author: Mike Snitzer

drivers/md/dm-thin.c

@@ -401,8 +401,7 @@ static int issue_discard(struct discard_op *op, dm_block_t data_b, dm_block_t da
 	sector_t s = block_to_sectors(tc->pool, data_b);
 	sector_t len = block_to_sectors(tc->pool, data_e - data_b);
 
-	return __blkdev_issue_discard(tc->pool_dev->bdev, s, len, GFP_NOWAIT,
-				      &op->bio);
+	return __blkdev_issue_discard(tc->pool_dev->bdev, s, len, GFP_NOIO, &op->bio);
 }
 
 static void end_discard(struct discard_op *op, int r)