netfilter: nf_log: add packet logging for netdev family

Move layer 2 packet logging into nf_log_l2packet() that resides in
nf_log_common.c, so this can be shared by both bridge and netdev
families.

This patch adds the boiler plate code to register the netdev logging
family.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>