exfat: fix incorrect update of stream entry in __exfat_truncate()

At truncate, there is a problem of incorrect updating in the file entry
pointer instead of stream entry. This will cause the problem of
overwriting the time field of the file entry to new_size. Fix it to
update stream entry.

Fixes: 98d917047e8b ("exfat: add file operations")
Cc: stable@vger.kernel.org # v5.7
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Git-commit: 29bbb14bfc80dd760b07d2be0a27e610562982e3
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: lixiang <lixiang@codeaurora.org>