mm/z3fold: minor clean up for z3fold_free

Use put_z3fold_header() to pair with get_z3fold_header.  Also fix the
wrong comments.  Minor readability improvement.

Link: https://lkml.kernel.org/r/20220308134311.59086-4-linmiaohe@huawei.com


Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-by: Vitaly Wool <vitaly.wool@konsulko.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>