[NET]: Make the loopback device per network namespace.

This patch makes loopback_dev per network namespace.  Adding
code to create a different loopback device for each network
namespace and adding the code to free a loopback device
when a network namespace exits.

This patch modifies all users the loopback_dev so they
access it as init_net.loopback_dev, keeping all of the
code compiling and working.  A later pass will be needed to
update the users to use something other than the initial network
namespace.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/decnet/dn_dev.c b/net/decnet/dn_dev.c
index bcaf4c5..26130af 100644
--- a/net/decnet/dn_dev.c
+++ b/net/decnet/dn_dev.c
@@ -869,10 +869,10 @@
 		rv = dn_dev_get_first(dev, addr);
 		read_unlock(&dev_base_lock);
 		dev_put(dev);
-		if (rv == 0 || dev == loopback_dev)
+		if (rv == 0 || dev == init_net.loopback_dev)
 			return rv;
 	}
-	dev = loopback_dev;
+	dev = init_net.loopback_dev;
 	dev_hold(dev);
 	goto last_chance;
 }