[NETNS]: Make the __addrconf_sysctl_register return an error

This error code will be needed to abort the namespace
creation if needed.

Probably, this is to be checked when a new device is
created (currently it is ignored).

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