fat: clean up xlate_to_uni()

xlate_to_uni() is called by vfat_build_slots() with sbi->nls_io as the
final argument.  nls_io can never be null at this point because the
check is already being done in fat_fill_super() wherein the mount fails
if it is null.

Signed-off-by: Namjae Jeon <linkinjeon@gmail.com>
Signed-off-by: Ravishankar N <cyberax82@gmail.com>
Acked-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>