MINOR: pattern/ip: simplify pat_match_ip() function

MINOR: pattern/ip: simplify pat_match_ip() function

pat_match_ip() has been updated several times over the last decade to
introduce new features, but it was never cleaned up.

The result is that the function is pretty hard to read, and there are
multiple duplicated code blocks so it becomes error-prone to maintain it,
plus it bloats the haproxy binary for nothing.

In this patch, we move the tree search (ip4 / ip6) logic into 2
dedicated helper functions. This allows us to refactor pat_match_ip()
without touching to the original behavior.