rgw: disable RGWDataChangesLog::add_entry() when log_data is off

rgw: disable RGWDataChangesLog::add_entry() when log_data is off

this restores a check for RGWZone::log_data in add_entry(). with
per-bucket replication, this check was replaced by a call to
`RGWBucketSyncPolicyHandler::bucket_exports_data()`

this call has to consult two rados objects,
`bucket.sync-source-hints.<bucketname>` and
`bucket.sync-target-hints.<bucketname>`

but if the zone is not configured for multisite, we should avoid these
extra object reads and return early

Fixes: https://tracker.ceph.com/issues/61300
Signed-off-by: Casey Bodley <cbodley@redhat.com>