Backport of this xorg-xserver commit:

  commit 75c51c67b340548286efd41a53882e2acaf74ab5
  Author: Alan Coopersmith <alan.coopersmith@sun.com>
  Date:   Thu Jun 18 09:49:12 2009 -0700

    Clarify use of and need for mffs vs. ffs
Signed-off-by: Alan Coopersmith <alan.coopersmith@sun.com>