[NET]: Make AF_PACKET handle multiple network namespaces

This is done by making packet_sklist_lock and packet_sklist per
network namespace and adding an additional filter condition on
received packets to ensure they came from the proper network
namespace.

Changes from v1:
- prohibit to call inet_dgram_ops.ioctl in other than init_net

Signed-off-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/net_namespace.h

@@ -32,6 +32,10 @@ struct net {
 	struct hlist_head	*dev_index_head;
 
 	struct sock 		*rtnl;			/* rtnetlink socket */
+
+	/* List of all packet sockets. */
+	rwlock_t		packet_sklist_lock;
+	struct hlist_head	packet_sklist;
 };
 
 #ifdef CONFIG_NET