fs/zonefs: Use the enum req_op type for tracing request operations

Improve static type checking by using the enum req_op type for request
operations.

Reviewed-by: Johannes Thumshirn <jth@kernel.org>
Reviewed-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Cc: Naohiro Aota <naohiro.aota@wdc.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20220714180729.1065367-64-bvanassche@acm.org


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