netfilter: nf_log: do not assume ethernet header in netdev family

In netdev family, we will handle non ethernet packets, so using
eth_hdr(skb)->h_proto is incorrect.

Meanwhile, we can use socket(AF_PACKET...) to sending packets, so
skb->protocol is not always set in bridge family.

Add an extra parameter into nf_log_l2packet to solve this issue.

Fixes: 1fddf4ba ("netfilter: nf_log: add packet logging for netdev family")
Signed-off-by: Liping Zhang <zlpnobody@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>