null_blk: fix checking for REQ_FUA

null_handle_bio() erroneously uses the bio_op macro
which masks respective request flag bits including REQ_FUA
out thus failing the check.

Fix by checking bio->bi_opf directly.

Signed-off-by: Heinz Mauelshagen <heinzm@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>