blk-mq: add bounds check on tag-to-rq conversion

We need to check for a valid index before accessing the array
element to avoid accessing invalid memory regions.
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NJeff Moyer <jmoyer@redhat.com>

Modified by Jens to drop the unlikely(), and make the fall through
path be having a valid tag.
Signed-off-by: NJens Axboe <axboe@fb.com>

block/blk-mq.c

@@ -544,7 +544,10 @@ EXPORT_SYMBOL(blk_mq_abort_requeue_list);
 
 struct request *blk_mq_tag_to_rq(struct blk_mq_tags *tags, unsigned int tag)
 {
+	if (tag < tags->nr_tags)
 		return tags->rqs[tag];
+
+	return NULL;
 }
 EXPORT_SYMBOL(blk_mq_tag_to_rq);