migrate_pages: split unmap_and_move() to _unmap() and _move()

This is a preparation patch to batch the folio unmapping and moving.

In this patch, unmap_and_move() is split to migrate_folio_unmap() and
migrate_folio_move().  So, we can batch _unmap() and _move() in different
loops later.  To pass some information between unmap and move, the
original unused dst->mapping and dst->private are used.

Link: https://lkml.kernel.org/r/20230213123444.155149-5-ying.huang@intel.com
Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Reviewed-by: Xin Hao <xhao@linux.alibaba.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Bharata B Rao <bharata@amd.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/migrate.h

@@ -18,6 +18,7 @@ struct migration_target_control;
  * - zero on page migration success;
  */
 #define MIGRATEPAGE_SUCCESS		0
+#define MIGRATEPAGE_UNMAP		1
 
 /**
  * struct movable_operations - Driver page migration