fat: don't use free_clusters for fat32

It seems that the recent Windows changed specification, and it's
undocumented.  Windows doesn't update ->free_clusters correctly.

This patch doesn't use ->free_clusters by default.  (instead, add "usefree"
for forcing to use it)

Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Cc: Juergen Beisert <juergen127@kreuzholzen.de>
Cc: Andreas Schwab <schwab@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>