This patch fixes a bug noticed by Al Viro:

   However, we still have a problem here - just what would
   happen if vfsmount is detached while we were grabbing namespace
   semaphore?  Refcount alone is not useful here - we might be held by
   whoever had detached the vfsmount.  IOW, we should check that it's
   still attached (i.e. that mnt->mnt_parent != mnt).  If it's not -
   just leave it alone, do mntput() and let whoever holds it deal with
   the sucker.  No need to put it back on lists.
Signed-off-by: Miklos Szeredi <miklos@szeredi.hu>
Cc: <viro@parcelfarce.linux.theplanet.co.uk>
Acked-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>