block/rnbd: Use blk_opf_t where appropriate

Improve static type checking by using the new blk_opf_t type to represent
the combination of a request and request flags.

Acked-by: Jack Wang <jinpu.wang@ionos.com>
Cc: Md. Haris Iqbal <haris.iqbal@ionos.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20220714180729.1065367-17-bvanassche@acm.org


Signed-off-by: Jens Axboe <axboe@kernel.dk>