block/brd: Use the enum req_op type

Improve static type checking by using the enum req_op type for a
function argument that represents a request operation.

Cc: Christoph Hellwig <hch@lst.de>
Cc: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20220714180729.1065367-13-bvanassche@acm.org


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