mlxsw: spectrum_router: Extract mlxsw_sp_rt6_is_gateway()

IPv6 counterpart of the previous patch: introduce a function to
determine whether a given route is a gateway route.

The new function takes a mlxsw_sp argument which follow-up patches will
use. Thus mlxsw_sp_fib6_entry_type_set() got that argument 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>