net: dsa: support VLAN filtering switchdev attr

When a user explicitly requests VLAN filtering with something like:

    # echo 1 > /sys/class/net/<bridge>/bridge/vlan_filtering

Switchdev propagates a SWITCHDEV_ATTR_ID_BRIDGE_VLAN_FILTERING port
attribute.

Add support for it in the DSA layer with a new port_vlan_filtering
function to let drivers toggle 802.1Q filtering on user demand.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>