[NETNS]: Don't panic on creating the namespace's loopback

When the loopback device is failed to initialize inside the new 
namespaces, panic() is called. Do not do it when the namespace 
in question is not the init_net.

Plus cleanup the error path a bit.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>