From: Bjorn Stenberg <bjorn@haxx.se>
      OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>

This simple patch makes the linux fat filesystem driver use the
next_cluster field in the fat_boot_fsinfo structure.  This field is a hint
where to start looking for free clusters.

Using this field makes a big difference for disks connected over slow links
such as USB 1.1.  Finding the first free cluster on a 40gig fat-formatted
usb disk can today take several minutes.  This patch cuts it down to a
fraction of a second.

Also, commit the next_cluster search hint toand from the superblock in
write_super/fill_super.