net: 8021q: resolve forwarding path for vlan devices

Add .ndo_fill_forward_path for vlan devices.

For instance, assuming the following topology:

                   IP forwarding
                  /             \
            eth0.100             eth0
            |
            eth0
            .
            .
            .
           ethX
     ab:cd:ef:ab:cd:ef

For packets going through IP forwarding to eth0.100 whose destination
MAC address is ab:cd:ef:ab:cd:ef, dev_fill_forward_path() provides the
following path:

        eth0.100 -> eth0

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>