staging: exfat: Clean up the namespace pollution part 6

staging: exfat: Clean up the namespace pollution part 6

Move a few more things so we can make them static and clear exfat.h out.
At this point, pretty much everything that can be static is static.
(Note: FAT_sync(), buf_sync(), and sync_alloc_bitmap() aren't called
anyplace, but aren't static because (a) that will toss an error and
(b) they probably *should* be getting called someplace

Signed-off-by: Valdis Kletnieks <Valdis.Kletnieks@vt.edu>
Link: https://lore.kernel.org/r/20191112211238.156490-11-Valdis.Kletnieks@vt.edu
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>