block: merge: get the 1st and last bvec via helpers

block: merge: get the 1st and last bvec via helpers

This patch applies the two introduced helpers to
figure out the 1st and last bvec.

Reviewed-by: Sagi Grimberg <sagig@mellanox.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
(cherry picked from commit e827091cb1bcd8e718ac3657845fb809c0b93324)

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>