[NET]: Make packet reception network namespace safe

This patch modifies every packet receive function
registered with dev_add_pack() to drop packets if they
are not from the initial network namespace.

This should ensure that the various network stacks do
not receive packets in a anything but the initial network
namespace until the code has been converted and is ready
for them.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/arp.c b/net/ipv4/arp.c
index 78dd344..bde1297 100644
--- a/net/ipv4/arp.c
+++ b/net/ipv4/arp.c
@@ -932,6 +932,9 @@
 {
 	struct arphdr *arp;
 
+	if (dev->nd_net != &init_net)
+		goto freeskb;
+
 	/* ARP header, plus 2 device addresses, plus 2 IP addresses.  */
 	if (!pskb_may_pull(skb, (sizeof(struct arphdr) +
 				 (2 * dev->addr_len) +