vfat vs msdos and -o flush

[Christoph Hellwig <hch@lst.de>]

Commit ae78bf9c4f5fde3c67e2829505f195d7347ce3e4 added a -o flush
option to the fat driver back in 2006, which causes asynchronous
writeout after I/O operations ASAP.  This already is quite hacky
and not something I like very much, but even worse the option
is accepted when using the more common vfat filesysten type, but
only actually implemented for the less common legacy msdos
filesystem type.

This tells us two lessons:

  - the -o flush option probably never got a whole lot of exposure
    and users didn't notice it doesn't work.  We might as well just
    remove it again
  - having the highlevel inode operations duplicated between msdos
    and vfat is probably a bad idea, and we should only branch out
    for the very low-level directory entry manipulations.