netfilter: Make nflog quiet when no one listen in userspace.

The message "nf_log_packet: can't log since no backend logging module loaded
in! Please either load one, or disable logging explicitly" was displayed for
each logged packet when no userspace application is listening to nflog events.
The message seems to warn for a problem with a kernel module missing but as
said before this is not the case. I thus propose to suppress the message (I
don't see any reason to flood the log because a user application has crashed.)

Signed-off-by: Eric Leblond <eric@inl.fr>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>