exfat: Set the unused characters of FileName field to the value 0000h

Some fsck tool complain that padding part of the FileName field
is not set to the value 0000h. So let's maintain filesystem cleaner,
as exfat's spec. recommendation.

Signed-off-by: Hyeongseok.Kim <Hyeongseok@gmail.com>
Reviewed-by: Sungjong Seo <sj1557.seo@samsung.com>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>