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: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NFlorian Westphal <fw@strlen.de>
Signed-off-by: NPablo Neira Ayuso <pablo@netfilter.org>

include/linux/netfilter_netdev.h

@@ -101,7 +101,11 @@ static inline struct sk_buff *nf_hook_egress(struct sk_buff *skb, int *rc,
 	nf_hook_state_init(&state, NF_NETDEV_EGRESS,
 			   NFPROTO_NETDEV, dev, NULL, NULL,
 			   dev_net(dev), NULL);
+
+	/* nf assumes rcu_read_lock, not just read_lock_bh */
+	rcu_read_lock();
 	ret = nf_hook_slow(skb, &state, e, 0);
+	rcu_read_unlock();
 
 	if (ret == 1) {
 		return skb;