netfilter: nfnetlink_log: fix silly refcount leak

Quick fix for memory/module refcount leak.
Reference count of listener instance never reaches 0.

Start/stop of ulogd2 is enough to trigger this bug!

Now, refcounting there looks very fishy in particular this code:

 	if (!try_module_get(THIS_MODULE)) {
		...

and creation of listener instance with refcount 2,
so it may very well be ripped and redone.  :-)

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>