exfat: Simplify exfat_utf8_d_cmp() for code points above U+FFFF

If two Unicode code points represented in UTF-16 are different then also
their UTF-32 representation must be different. Therefore conversion from
UTF-32 to UTF-16 is not needed.

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>