nfp: add VLAN filtering support

Add general use per-vNIC mailbox area and use it for VLAN filtering
support.  Initially proto is hardcoded to 802.1q.

Signed-off-by: Pablo Cascón <pablo.cascon@netronome.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>