s390: use RCU to walk list of network devices

This is similar to other cases where for_each_netdev_rcu
can be used when gathering information.

By inspection, don't have platform or cross-build environment
to validate.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>