net: if: Add special handling for IPv4/6 address check for VPN

This is a hack that is used until we have proper IP routing
in place. The code has now special check that makes sure that
we only route IP packets to VPN interface when the packet is
destined to that subnet. So if destination IP address does
not belong to VPN interface subnet, it is not routed there.

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