Add general use per-vNIC mailbox area and use it for VLAN filtering
support.  Initially proto is hardcoded to 802.1q.
Signed-off-by: NPablo Cascón <pablo.cascon@netronome.com>
Signed-off-by: NJakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>