block: don't optimize for non-cloned bio in bio_get_last_bvec()

For !BIO_CLONED bio, we can use .bi_vcnt safely, but it
doesn't mean we can just simply return .bi_io_vec[.bi_vcnt - 1]
because the start postion may have been moved in the middle of
the bvec, such as splitting in the middle of bvec.

Fixes: 7bcd79ac(block: bio: introduce helpers to get the 1st and last bvec)
Cc: stable@vger.kernel.org
Reported-by: NKent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: NMing Lei <ming.lei@canonical.com>
Signed-off-by: NJens Axboe <axboe@fb.com>

include/linux/bio.h

@@ -320,11 +320,6 @@ static inline void bio_get_last_bvec(struct bio *bio, struct bio_vec *bv)
 	struct bvec_iter iter = bio->bi_iter;
 	int idx;
 
-	if (!bio_flagged(bio, BIO_CLONED)) {
-		*bv = bio->bi_io_vec[bio->bi_vcnt - 1];
-		return;
-	}
-
 	if (unlikely(!bio_multiple_segments(bio))) {
 		*bv = bio_iovec(bio);
 		return;