net: Refactor IP checks just before sending packets

* Check IPv4 TTL or IPv6 hop limit and drop the packet if
  the value is 0
* Check the IP addresses so that we do the loopback check
  at runtime if the packet is destined to loopback interface.
* Update the statistics properly for dropped packets.
* Do not update sent packets if we drop packets.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>