Instead of defining a local version of struct ethhdr use the standard
definition from <linux/if_ether.h>.

The fields in the <linux/if_ether.h> definition have different names:
 - dest -> h_dest
 - src -> h_source
 - type -> h_proto

While there, use a few other standard functions/macros:
 - eth_broadcast_addr (instead of a memset)
 - ETH_ALEN
 - ETH_P_8021Q
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>