connector: Fix proc_event_num_listeners count not cleared

When we register a cn_proc listening event, the proc_event_num_listener
variable will be incremented by one, but if PROC_CN_MCAST_IGNORE is
not called, the count will not decrease.
This will cause the proc_*_connector function to take the wrong path.
It will reappear when the forkstat tool exits via ctrl + c.
We solve this problem by determining whether
there are still listeners to clear proc_event_num_listener.

Signed-off-by: wangkeqi <wangkeqiwang@didiglobal.com>
Signed-off-by: David S. Miller <davem@davemloft.net>