affs: rename local toupper() to fn() to avoid confusion

A compiler may see the collision with the toupper() defined in ctype.h:

 fs/affs/namei.c:159:19: warning: unused variable 'toupper' [-Wunused-variable]
   159 |         toupper_t toupper = affs_get_toupper(sb);

To prevent this from happening, rename toupper local variable to fn.

Initially this had been introduced by 24579a881513 ("v2.4.3.5 -> v2.4.3.6")
in the history.git by history group.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>