dm/zone: Use the enum req_op type

Use the enum req_op type for request operations instead of unsigned int.
This patch fixes a sparse warning that has been introduced by making
enum req_op __bitwise.

Reviewed-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20220714180729.1065367-30-bvanassche@acm.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>