When handling long file names directory entries may be split over multiple
clusters. We must make sure that new clusters are zero filled on disk.
When allocating a new cluster for a directory flush it.
The flushing should be executed before updating the FAT. This way if
flushing fails, we still have a valid directory structure.
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>