block: fix build warning in merging bvecs

Commit f6970f83 ("block: don't check if adjacent bvecs in one bio can
be mergeable") changes bvec merge by only considering two bvecs from
different bios. However, if the former bio doesn't inlcude any io bvec,
then the following warning may be triggered:

 warning: ‘bvec.bv_offset’ may be used uninitialized in this function [-Wmaybe-uninitialized]

In practice, it shouldn't be triggered.

Fixes it by adding check on former bio, the check shouldn't add any cost
given 'bio->bi_iter' can be hit in cache.
Reported-by: Jens Axboe <axboe@kernel.dk>
Fixes: f6970f83 ("block: don't check if adjacent bvecs in one bio can be mergeable")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-merge.c

@@ -353,7 +353,7 @@ EXPORT_SYMBOL(blk_queue_split);
 static unsigned int __blk_recalc_rq_segments(struct request_queue *q,
 					     struct bio *bio)
 {
-	struct bio_vec bv, bvprv = { NULL };
+	struct bio_vec uninitialized_var(bv), bvprv = { NULL };
 	unsigned int seg_size, nr_phys_segs;
 	unsigned front_seg_size;
 	struct bio *fbio, *bbio;
@@ -400,8 +400,10 @@ static unsigned int __blk_recalc_rq_segments(struct request_queue *q,
 			new_bio = false;
 		}
 		bbio = bio;
-		bvprv = bv;
-		new_bio = true;
+		if (likely(bio->bi_iter.bi_size)) {
+			bvprv = bv;
+			new_bio = true;
+		}
 	}
 
 	fbio->bi_seg_front_size = front_seg_size;
@@ -527,7 +529,7 @@ static int __blk_bios_map_sg(struct request_queue *q, struct bio *bio,
 			     struct scatterlist *sglist,
 			     struct scatterlist **sg)
 {
-	struct bio_vec bvec, bvprv = { NULL };
+	struct bio_vec uninitialized_var(bvec), bvprv = { NULL };
 	struct bvec_iter iter;
 	int nsegs = 0;
 	bool new_bio = false;
@@ -550,8 +552,10 @@ static int __blk_bios_map_sg(struct request_queue *q, struct bio *bio,
  next_bvec:
 			new_bio = false;
 		}
-		bvprv = bvec;
-		new_bio = true;
+		if (likely(bio->bi_iter.bi_size)) {
+			bvprv = bvec;
+			new_bio = true;
+		}
 	}
 
 	return nsegs;