staging/lustre: Update cfs_cpu_notify switch statement with a comment

We do really mean to fall through to that default case statement from
all previous ones, so add a comment to unconfuse verious tools

Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>