af_packet: Introduce egress hook

Add egress hook for AF_PACKET sockets that have the PACKET_QDISC_BYPASS
socket option set to on, which allows packets to escape without being
filtered in the egress path.

This patch only updates the AF_PACKET path, it does not update
dev_direct_xmit() so the XDP infrastructure has a chance to bypass
Netfilter.

[lukas: acquire rcu_read_lock, fix typos, rebase]
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>