exfat: fix the new buffer was not zeroed before writing

Before writing, if a buffer_head marked as new, its data must
be zeroed, otherwise uninitialized data in the page cache will
be written.

So this commit uses folio_zero_new_buffers() to zero the new
buffers before ->write_end().

Fixes: 6630ea49 ("exfat: move extend valid_size into ->page_mkwrite()")
Reported-by:  <syzbot+91ae49e1c1a2634d20c0@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=91ae49e1c1a2634d20c0


Tested-by:  <syzbot+91ae49e1c1a2634d20c0@syzkaller.appspotmail.com>
Signed-off-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
Reviewed-by: Sungjong Seo <sj1557.seo@samsung.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>