MINOR: dns: fix wrong score computation in dns_get_ip_from_response

MINOR: dns: fix wrong score computation in dns_get_ip_from_response

dns_get_ip_from_response() is used to compare the caller current IP to
the IP available in the records returned by the DNS server.
A scoring system is in place to get the best IP address available.
That said, in the current implementation, there are a couple of issues:
1. a comment does not match what the code does
2. the code does not match what the commet says (score value is not
   incremented with '2')

This patch fixes both issues.

Backport status: 1.8