cn: verify msg->len before making callback

The struct cn_msg len field comes from userspace and needs to be
validated.  More logical to do so here where the cn_msg pointer is
pulled out of the sk_buff than the callback which is passed cn_msg *
and might assume no validation is needed.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Evgeniy Polyakov <zbr@ioremap.net>
Signed-off-by: David Fries <David@Fries.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>