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>