exfat: fix wrong size update of stream entry by typo

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>