net/bpfilter: fix dprintf usage for /dev/kmsg

The bpfilter UMH code was recently changed to log its informative messages to
/dev/kmsg, however this interface doesn't support SEEK_CUR yet, used by
dprintf(). As result dprintf() returns -EINVAL and doesn't log anything.

However there already had some discussions about supporting SEEK_CUR into
/dev/kmsg interface in the past it wasn't concluded. Since the only user of
that from userspace perspective inside the kernel is the bpfilter UMH
(userspace) module it's better to correct it here instead waiting a conclusion
on the interface.

Fixes: 36c4357c

 ("net: bpfilter: print umh messages to /dev/kmsg")
Signed-off-by: Bruno Meneguele <bmeneg@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>