[ Upstream commit 2d62e6b0 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>