mm: compaction: introduce map_pages()

mm: compaction: introduce map_pages()

This commit creates a map_pages() function which map pages freed
using split_free_pages().  This merely moves some code from
isolate_freepages() so that it can be reused in other places.

Signed-off-by: Michal Nazarewicz <[email protected]>
Signed-off-by: Marek Szyprowski <[email protected]>
Acked-by: Mel Gorman <[email protected]>
Reviewed-by: KAMEZAWA Hiroyuki <[email protected]>
Tested-by: Robert Nelson <[email protected]>
Tested-by: Barry Song <[email protected]>