mlxsw: spectrum_fid: Store ifindex of NVE device in FID

The driver periodically polls for new FDB entries learned by the device.
In the case of an FDB entry learned from a VxLAN tunnel, the
notification includes the IP of the remote VTEP, the filtering
identifier (FID) and the source MAC address of the overlay packet.

Assuming learning is enabled in the VxLAN and bridge drivers, the driver
needs to generate a notification and update them about the new FDB
entry.

Store the ifindex of the NVE device in the FID so that the driver will
be able to update the VxLAN and bridge drivers using it.

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