genetlink: add function genl_has_listeners()

This function is the counterpart of the function netlink_has_listeners().

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>