block: export blk_verify_command for SG v4

blk_fill_sghdr_rq doesn't work for SG v4 so verify_command needed to
be exported.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>