BUG/MEDIUM: resolvers: fix name compression pointer validation in resolv_read_name()

BUG/MEDIUM: resolvers: fix name compression pointer validation in resolv_read_name()

The original DNS code would only use the 8 lower bits of the compression
offset. This was fixed in 2.0 with commit 2fa66c3b9 ("BUG/MEDIUM: dns:
overflowed dns name start position causing invalid dns error") but it was
not sufficient because the anti-loop check continues to use only 8 of the
14 bits, thus a crafted response where the 8 lower bits pass the check and
the 6 higher should fail it would be accepted. The impacts remains limited
thanks to the bounds check and the recursion limits, but such invalid
responses could still cost a lot to process. Let's compute the 14-bit
offset once for all and use it everywhere.