The stream.size field is updated to the value of create timestamp
of the file entry. Fix this to use correct stream entry pointer.

Fixes: 29bbb14b ("exfat: fix incorrect update of stream entry in __exfat_truncate()")
Signed-off-by: Hyeongseok Kim <hyeongseok@gmail.com>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>