fix: dev: Make dns_validator_cancel() respect the data ownership

There was a data race dns_validator_cancel() was called when the
offloaded operations were in progress.  Make dns_validator_cancel()
respect the data ownership and only set new .canceling variable when
the offloaded operations are in progress.  The cancel operation would
then finish when the offloaded work passes the ownership back to the
respective thread.

Closes #4926

Merge branch '4926-fix-data-race-in-dns_validator' into 'main'

See merge request isc-projects/bind9!9470