um: Use enum req_op where appropriate

Improve static type checking by using type enum req_op instead of int where
appropriate.

Cc: Richard Weinberger <richard@nod.at>
Cc: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20220714180729.1065367-21-bvanassche@acm.org


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