ipv4: Send a netevent whenever multipath hash policy is changed

Devices performing IPv4 forwarding need to update their multipath hash
policy whenever it is changed.

Inform these devices by generating a netevent.

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