Fix extra detach when dns_validator create_fetch() detects deadlock

When create_fetch() in the dns_validator unit detects deadlock, it returns DNS_R_NOVALIDSIG, but it didn't attach to the validator. The other condition to returning result != ISC_R_SUCCESS would be error from dns_resolver_createfetch(). The caller (in two places out of three) would detect the error condition and always detach from the validator.

Move the dns_validator_detach() on dns_resolver_createfetch() error condition to create_fetch() function and cleanup the extra detaches in seek_dnskey() and get_dsset().

Closes #4115 (closed)