netfilter: nf_log: incorrect offset to network header

netfilter: nf_log: incorrect offset to network header

NFPROTO_ARP is expecting to find the ARP header at the network offset.

In the particular case of ARP, HTYPE= field shows the initial bytes of
the ethernet header destination MAC address.

 netdev out: IN= OUT=bridge0 MACSRC=c2:76:e5:71:e1:de MACDST=36:b0:4a:e2:72:ea MACPROTO=0806 ARP HTYPE=14000 PTYPE=0x4ae2 OPCODE=49782

NFPROTO_NETDEV egress hook is also expecting to find the IP headers at
the network offset.

Fixes: 363ce390b061 ("netfilter: add generic ARP packet logger")
Reported-by: Tom Yan <tom.ty89@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>