net: pkt: Cleanup validation of min fragment size based on max headers

1. Where we calculate max size, name variable (preprocessor define)
correspondingly.
2. Calling TCP/UDP an "app protocol" is original, use "next protocol"
terminology of IPv6.
3. As headers go as IP, then "next", order calculations that way too.
4. Add more comments.

Addresses: #8723

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