null_blk: fix passing of REQ_FUA flag in null_handle_rq

REQ_FUA should be checked using rq->cmd_flags instead of req_op().

Fixes: deb78b41 ("nullb: emulate cache")
Signed-off-by: Hou Pu <houpu@bytedance.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>