MINOR: dns: proper domain name validation when receiving DNS response

MINOR: dns: proper domain name validation when receiving DNS response

The analyse of CNAME resolution and request's domain name was performed
twice:
- when validating the response buffer
- when loading the right IP address from the response

Now DNS response are properly loaded into a DNS response structure, we
do the domain name validation when loading/validating the response in
the DNS strcucture and later processing of this task is now useless.

backport: no