mlxsw currently does not support v6 gateways with v4 routes. Commit
19a9d136 ("ipv4: Flag fib_info with a fib_nh using IPv6 gateway")
prevents a route from being added, but nothing stops the replace or
append. Add a catch for them too.
    $ ip  ro add 172.16.2.0/24 via 10.99.1.2
    $ ip  ro replace 172.16.2.0/24 via inet6 fe80::202:ff:fe00:b dev swp1s0
    Error: mlxsw_spectrum: IPv6 gateway with IPv4 route is not supported.
    $ ip  ro append 172.16.2.0/24 via inet6 fe80::202:ff:fe00:b dev swp1s0
    Error: mlxsw_spectrum: IPv6 gateway with IPv4 route is not supported.
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>