vxlan: Add switchdev notifications

When offloading VXLAN devices, drivers need to know about events in
VXLAN FDB database. Since VXLAN models a bridge, it is natural to
distribute the VXLAN FDB notifications using the pre-existing switchdev
notification mechanism.

To that end, introduce two new notification types:
SWITCHDEV_VXLAN_FDB_ADD_TO_DEVICE and SWITCHDEV_VXLAN_FDB_DEL_TO_DEVICE.
Introduce a new function, vxlan_fdb_switchdev_call_notifiers() to send
the new notifier types, and a struct switchdev_notifier_vxlan_fdb_info
to communicate the details of the FDB entry under consideration.

Invoke the new function from vxlan_fdb_notify().

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>