[NETFILTER]: xt_pkttype: Add explicit check for IPv4

In the PACKET_LOOPBACK case, the skb data was always interpreted as
IPv4, but that is not valid for IPv6, obviously. Fix this by adding an
extra condition to check for AF_INET.

Signed-off-by: Jan Engelhardt <jengelh@computergmbh.de>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>