This changes ->free_clusters and ->prev_free from "int" to "unsigned int". 
These value should be never negative value (but it's using 0xffffffff(-1) as
undefined state).

With this changes, fatfs would handle the corruption of free_clusters
more proper.
Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>