[NET]: Proper comment for loopback initialization order.

Loopback device is special. It should be initialized at the very
beginning.  Initialization order has been changed by
Eric W. Biederman <ebiederm@xmission.com> and this change is non-obvious
and important enough to add proper comment.
Signed-off-by: NDenis V. Lunev <den@openvz.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/loopback.c

@@ -293,4 +293,7 @@ static int __init loopback_init(void)
 	return register_pernet_device(&loopback_net_ops);
 }
 
+/* Loopback is special. It should be initialized before any other network
+ * device and network subsystem.
+ */
 fs_initcall(loopback_init);