mlxsw: spectrum_router: Extract __mlxsw_sp_ipip_entry_update_tunnel()

The work that's done by mlxsw_sp_netdevice_ipip_ol_vrf_event() is a good
basis for a more versatile function that would take care of all sorts of
tunnel updates requests: __mlxsw_sp_ipip_entry_update_tunnel(). Extract
that function. Factor out a helper mlxsw_sp_ipip_entry_ol_lb_update() as
well.

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