mm/z3fold: remove unused function handle_to_z3fold_header()

handle_to_z3fold_header() is unused now.  So we can remove it.  As a
result, get_z3fold_header() becomes the only caller of
__get_z3fold_header() and the argument lock is always true.  Therefore we
could further fold the __get_z3fold_header() into get_z3fold_header() with
lock = true.

Link: https://lkml.kernel.org/r/20210619093151.1492174-5-linmiaohe@huawei.com
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-by: Vitaly Wool <vitaly.wool@konsulko.com>
Cc: Hillf Danton <hdanton@sina.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>