net: bpfilter: print umh messages to /dev/kmsg

bpfilter_umh currently printed all messages to /dev/console and this
might interfere the user activity(*).

This commit changes the output device to /dev/kmsg so that the messages
from bpfilter_umh won't show on the console directly.

(*) https://bugzilla.suse.com/show_bug.cgi?id=1140221

Signed-off-by: Gary Lin <glin@suse.com>
Signed-off-by: David S. Miller <davem@davemloft.net>