In dounmount(), only call checkdirs() prior to VFS_UNMOUNT() in the
forced unmount case. Otherwise, a file system that is referenced
only by process fd_cdir/fd_rdir references to the file system root
vnode will be successfully unmounted without the MNT_FORCE flag.

The previous behaviour was not compatible with the unmount semantics
required by amd(8), so file systems could be unexpectedly unmounted
while there were still references to the file system root directory.

Reported by: Erez Zadok <ezk@cs.sunysb.edu>
Approved by: re (scottl)