This fixes some issues reported on linux-cluster.  Essentially, the
    previous code was deadlocking in some cases, primarily, when you run
    the sync command.  That's because sync does a down_read on the
    s_umount semaphore.  The deadlock was caused by some code I removed
    with the port to the 2.6.20 kernel.  However, I removed it because
    s_umount was needed in the down state for unmounts.  That's because
    the diaper device isn't managed by the normal mount path in vfs.
    I also fixed a kernel panic I discovered in testing when trying to mount
    an invalid fs; for example, trying to mount a gfs2 file system as gfs.