net: dns: dns_pack: Revamp FQDN parsing in answers

Simplify algorithm to skip (aka calculate length) of encoded domain
name in a DNS answer. Now it's fully compliant to RFC 1035 regarding
handling of compressed FQDNs. Additionally, bounds checking is now
performed by the parsing code.

Fixes: #18334

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>