Instead of defining the local struct vlantag use the standard definition
of vlan_hdr from <linux/if_vlan.h>.

The fields in the <linux/if_vlan.h> definition have different names:
 - vlan -> h_vlan_TCI
 - subtype -> h_vlan_encapsulated_proto

While there, use also the ETH_P_IP macro instead of an hard-coded 0x0800
value.
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>