a directory to a subdir of the root directory from somewhere else.
For all directory moves that change the parent directory, the dotdot
entry must be fixed up. For msdosfs, the root directory is magic for
non-FAT32. It is less magic for FAT32, but needs the same magic for
the dotdot fixup. It didn't have it.
Both chkdsk and fsck_msdosfs fix the corrupt directory entries with no
problems.
The fix is to use the same magic for dotdot in msdosfs_rename() as in
msdosfs_mkdir().
For msdosfs_mkdir(), document the magic. When writing the dotdot entry
in mkdir, use explicitly set pcl variable instead on relying on the
start cluster of the root directory typically has a value < 65536.
Submitted by: bde
MFC after: 1 week
96b12145fb