net: core: Do IPv4/6 packet checks only for those packet types

The check_ip() in net_core.c did not check that the packet
Ethernet type is either IPv4 or IPv6. This meant that we for
example checked TTL also for ARP packets which is pointless as
those are not IPv4 packets.

Fix this by checking the link layer protocol type of the packet
to be either IPv4 or IPv6 before doing L3 checks.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>
(cherry picked from commit fc16c42b)