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

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: 7bcd79ac50d9(block: bio: introduce helpers to get the 1st and last bvec)
Cc: stable@vger.kernel.org
Reported-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
(cherry picked from commit 90d0f0f11588ec692c12f9009089b398be395184)

Orabug: 27775588

Signed-off-by: Shan Hai <shan.hai@oracle.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Brian Maly <brian.maly@oracle.com>