openvswitch: restore OVS_FLOW_CMD_NEW notifications

Since commit fb5d1e9e ("openvswitch: Build flow cmd netlink reply only if needed."),
the new flows are not notified to the listeners of OVS_FLOW_MCGROUP.

This commit fixes the problem by using the genl function, ie
genl_has_listerners() instead of netlink_has_listeners().

Signed-off-by: Samuel Gauthier <samuel.gauthier@6wind.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>