staging: exfat: Clean up the namespace pollution part 7

Global functions called 'buf*' are a linkage editor disaster waiting to
happen.  Rename our buf_* functions to exfat_buf_*

Signed-off-by: Valdis Kletnieks <Valdis.Kletnieks@vt.edu>
Link: https://lore.kernel.org/r/20191112211238.156490-12-Valdis.Kletnieks@vt.edu


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