mm/migration: remove some duplicated codes in migrate_pages

Remove the duplicated codes in migrate_pages to simplify the code.  Minor
readability improvement.  No functional change intended.

Link: https://lkml.kernel.org/r/20220318111709.60311-9-linmiaohe@huawei.com


Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: "Huang, Ying" <ying.huang@intel.com>
Cc: Muchun Song <songmuchun@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>