mlxsw: spectrum_nve: Add mlxsw_sp_nve_ops.fdb_clear_offload

If there are any offloaded FDB entries at an NVE device at the time that
it's un-offloaded, their offloaded marks need to be cleared. How that is
done depends on NVE device type, and therefore add a per-NVE-type
operation.

Implement the operation for the sole NVE device type currently supported
by mlxsw, VXLAN.

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>