From: Ian Kent <raven@themaw.net>

This patch is the result of an e-mail discussion with Soni Maneesh.  He felt
that the use of reference counts in the expire module is unreliable (in the
presence of rcu) and suggested it should use standard VFS calls where
possible.  This has been done.  Once the boundary in autofs is reached we
have no choice but to resort using reference counts (but under the
vfsmount_lock).


After review by hch:

- renamed autofs4_may_umount to __may_umount_tree, made it static and moved
  it to namespace.c.

- added stub function may_umount_tree with description

- altered may_umount to use above stub function and added little description

- added may_umount_tree prototype to fs.h

- removed the EXPORT_SYMBOL for vfsmount_lock

- updated expire.c to suit