netfilter: egress: silence egress hook lockdep splats

Netfilter assumes its called with rcu_read_lock held, but in egress
hook case it may be called with BH readlock.

This triggers lockdep splat.

In order to avoid to change all rcu_dereference() to
rcu_dereference_check(..., rcu_read_lock_bh_held()), wrap nf_hook_slow
with read lock/unlock pair.

Reported-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>