Addresse a problem where stale dentrys stop mounts from happening.

When a mount point directory is pre-created and a non-existent entry within it
is requested a dentry ends up being created within the mount point directory
which stops future mounts.  The problem is solved by ignoring negative,
unhashed dentrys in the mount point d_subdirs list.

Additionally the apparent cacheing of -ENOENT returns from requests is
removed.  The test on d_time is a tautology and d_time is not initialised and
has an unexpected value.  In short it doesn't do what it's meant to.

The cacheing of failed requests to the daemon is important and will be
followed up later.

Signed-off-by: Ian Kent <raven@themaw.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>