vxlan: Support marking RDSTs as offloaded

Offloaded bridge FDB entries are marked with NTF_OFFLOADED. Implement a
similar mechanism for VXLAN, where a given remote destination can be
marked as offloaded.

To that end, introduce a new event, SWITCHDEV_VXLAN_FDB_OFFLOADED,
through which the marking is communicated to the vxlan driver. To
identify which RDST should be marked as offloaded, an
switchdev_notifier_vxlan_fdb_info is passed to the listeners. The
"offloaded" flag in that object determines whether the offloaded mark
should be set or cleared.

When sending offloaded FDB entries over netlink, mark them with
NTF_OFFLOADED.

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>