Fix bug in blk_rq_merge_ok

commit 7ee8e4f3 upstream.

Use the right array index to reference the last
element of rq->biotail->bi_io_vec[]
Signed-off-by: Wenbo Wang <wenbo.wang@memblaze.com>
Reviewed-by: Chong Yuan <chong.yuan@memblaze.com>
Fixes: 66cb45aa ("block: add support for limiting gaps in SG lists")
Signed-off-by: Jens Axboe <axboe@fb.com>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

block/blk-merge.c

@@ -596,7 +596,7 @@ bool blk_rq_merge_ok(struct request *rq, struct bio *bio)
 	if (q->queue_flags & (1 << QUEUE_FLAG_SG_GAPS)) {
 		struct bio_vec *bprev;
 
-		bprev = &rq->biotail->bi_io_vec[bio->bi_vcnt - 1];
+		bprev = &rq->biotail->bi_io_vec[rq->biotail->bi_vcnt - 1];
 		if (bvec_gap_to_prev(bprev, bio->bi_io_vec[0].bv_offset))
 			return false;
 	}