block/zram: Use enum req_op where appropriate

Improve static type checking by using the enum req_op type where
appropriate.

Cc: Minchan Kim <minchan@kernel.org>
Cc: Nitin Gupta <ngupta@vflare.org>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20220714180729.1065367-19-bvanassche@acm.org


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