qeth: ensure that __vlan_find_dev_deep() is called with rcu_read_lock

Also benefit from rcu_read_lock held and use __in_dev_get_rcu() in ipv4 case.

Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>