It is overkill to update the FS_INFO whenever modifying
prev_free/free_clusters, because those are just a hint.

So, this patch uses ->write_super() for updating FS_INFO instead.
Signed-off-by: NOGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>