vlan: make vlan_dev_real_dev work over stacked vlans

Sometimes we might have stacked vlans on top of each other, and we're
interested in the first non-vlan real device on the path, so transform
vlan_dev_real_dev to go over the stacked vlans and extract the first
non-vlan device.

Signed-off-by: Nikolay Aleksandrov <nikolay@redhat.com>
Signed-off-by: Veaceslav Falico <vfalico@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>