s390/qeth: support net namespaces for L3 devices

Enable the L3 driver's IPv4 address notifier to watch for events on qeth
devices that have been moved into a net namespace. We need to program
those IPs into the HW just as usual, otherwise inbound traffic won't
flow.

Fixes: 6133fb1a ("[NETNS]: Disable inetaddr notifiers in namespaces other than initial.")
Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>