memory-hotplug: fix pages missed by race rather than failing

If race between allocation and isolation in memory-hotplug offline
happens, some pages could be in MIGRATE_MOVABLE of free_list although the
pageblock's migratetype of the page is MIGRATE_ISOLATE.

The race could be detected by get_freepage_migratetype in
__test_page_isolated_in_pageblock.  If it is detected, now EBUSY gets
bubbled all the way up and the hotplug operations fails.

But better idea is instead of returning and failing memory-hotremove, move
the free page to the correct list at the time it is detected.  It could
enhance memory-hotremove operation success ratio although the race is
really rare.

Suggested by Mel Gorman.

[[email protected]: small cleanup]
Signed-off-by: Minchan Kim <[email protected]>
Cc: KAMEZAWA Hiroyuki <[email protected]>
Reviewed-by: Yasuaki Ishimatsu <[email protected]>
Acked-by: Mel Gorman <[email protected]>
Cc: Xishi Qiu <[email protected]>
Cc: Wen Congyang <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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