drivers/staging/exfat - by default, prohibit mount of fat/vfat

Concerns have been raised about the exfat driver accidentally mounting
fat/vfat file systems.  Add an extra configure option to help prevent that.

Signed-off-by: Valdis Kletnieks <valdis.kletnieks@vt.edu>
Link: https://lore.kernel.org/r/245727.1567183359@turing-police


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>