vxlan: Add hardware FDB learning

In order to allow devices to signal learning events to VXLAN, introduce
two new switchdev messages: SWITCHDEV_VXLAN_FDB_ADD_TO_BRIDGE and
SWITCHDEV_VXLAN_FDB_DEL_TO_BRIDGE.

Listen to these notifications in the vxlan driver. The FDB entries
learned this way have an NTF_EXT_LEARNED flag, and only entries marked
as such can be unlearned by the _DEL_ event. They are also immediately
marked as offloaded. This is the same behavior that the bridge driver
observes.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>