mm: rename and move get/set_freepage_migratetype

The pair of get/set_freepage_migratetype() functions are used to cache
pageblock migratetype for a page put on a pcplist, so that it does not
have to be retrieved again when the page is put on a free list (e.g.
when pcplists become full).  Historically it was also assumed that the
value is accurate for pages on freelists (as the functions' names
unfortunately suggest), but that cannot be guaranteed without affecting
various allocator fast paths.  It is in fact not needed and all such
uses have been removed.

The last remaining (but pointless) usage related to pages of freelists
is in move_freepages(), which this patch removes.

To prevent further confusion, rename the functions to
get/set_pcppage_migratetype() and expand their description.  Since all
the users are now in mm/page_alloc.c, move the functions there from the
shared header.

Signed-off-by: Vlastimil Babka <[email protected]>
Acked-by: David Rientjes <[email protected]>
Acked-by: Joonsoo Kim <[email protected]>
Cc: Minchan Kim <[email protected]>
Acked-by: Michal Nazarewicz <[email protected]>
Cc: Laura Abbott <[email protected]>
Reviewed-by: Naoya Horiguchi <[email protected]>
Cc: Seungho Park <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: "Kirill A. Shutemov" <[email protected]>
Acked-by: Mel Gorman <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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