There is no documented methods to mark FAT as dirty.  Unofficially MS
started to use reserved Byte in boot sector for this purpose, at least
since Win 2000.  With Win 7 user is warned if fs is dirty and asked to
clean it.

Different versions of Win, handle it in different ways, but always have
same meaning:

- Win 2000 and XP, set it on write operations and
  remove it after operation was finnished
- Win 7, set dirty flag on first write and remove it on umount.

We will do it as follows:

- set dirty flag on mount. If fs was initially dirty, warn user,
  remember it and do not do any changes to boot sector.
- clean it on umount. If fs was initially dirty, leave it dirty.
- do not do any thing if fs mounted read-only.
- TODO: leave fs dirty if we found some error after mount.
Signed-off-by: NOleksij Rempel <bug-track@fisher-privat.net>
Signed-off-by: NOGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>