mm, page_alloc: avoid page_to_pfn() in move_freepages()

hulk inclusion
category: bugfix
bugzilla: 46856
CVE: NA

---------------------------

The start_pfn and end_pfn are already available in move_freepages_block(),
there is no need to go back and forth between page and pfn in move_freepages
and move_freepages_block, and pfn_valid_within() should validate pfn first
before touching the page.

Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Acked-by: Xie XiuQi <xiexiuqi@huawei.com>
Signed-off-by: Chen Jun <chenjun102@huawei.com>