ovl: fix tmpfile leak

Missed an error cleanup.

Reported-by:  <syzbot+fd749a7ea127a84e0ffd@syzkaller.appspotmail.com>
Fixes: 2b1a7746 ("ovl: use vfs_tmpfile_open() helper")
Cc: <stable@vger.kernel.org> # v6.1
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>