Alter the expire semantics that define how "busyness" is determined.
Currently a last_used counter is updated on every revalidate from processes
other than the mount owner process group.

This patch changes that so that an expire candidate is busy only if it has a
reference count greater than the expected minimum, such as when there is an
open file or working directory in use.

This method is the only way that busyness can be established for direct mounts
within the new implementation.  For consistency the expire semantic is made
the same for all mounts.

A side effect of the patch is that mounts which remain mounted unessessarily
in the presence of some GUI programs that scan the filesystem should now
expire.
Signed-off-by: Ian Kent <raven@themaw.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>