mlxsw: spectrum: Trap IPv4 packets with Router Alert option

In case local sockets have the IP_ROUTER_ALERT socket option set, then
they expect to get packets with the Router Alert option.

Trap such packets, so that the kernel could inspect them and potentially
send them to interested sockets.

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