dm/dm-zoned: Use the enum req_op type

Improve static type checking by using the enum req_op type for arguments
that represent a request operation.

Reviewed-by: Damien Le Moal <damien.lemoal@wdc.com>
Cc: Alasdair Kergon <agk@redhat.com>
Cc: Mike Snitzer <snitzer@kernel.org>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20220714180729.1065367-31-bvanassche@acm.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>