[NETFILTER]: nfnetlink_log: remove fallback to group 0

Don't fallback to group 0 if no instance can be found for the given group.
This potentially confuses the listener and is not what the user configured.
Also remove the ring buffer spamming that happens when rules are set up
before the logging daemon is started.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>