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When MIGRATE_UNMOVABLE pages are freed from MIGRATE_UNMOVABLE type
pageblock (and some MIGRATE_MOVABLE pages are left in it) waiting until an
allocation takes ownership of the block may take too long.  The type of
the pageblock remains unchanged so the pageblock cannot be used as a
migration target during compaction.

Fix it by:

* Adding enum compact_mode (COMPACT_ASYNC_[MOVABLE,UNMOVABLE], and
  COMPACT_SYNC) and then converting sync field in struct compact_control
  to use it.

* Adding nr_pageblocks_skipped field to struct compact_control and
  tracking how many destination pageblocks were of MIGRATE_UNMOVABLE type.
   If COMPACT_ASYNC_MOVABLE mode compaction ran fully in
  try_to_compact_pages() (COMPACT_COMPLETE) it implies that there is not a
  suitable page for allocation.  In this case then check how if there were
  enough MIGRATE_UNMOVABLE pageblocks to try a second pass in
  COMPACT_ASYNC_UNMOVABLE mode.

* Scanning the MIGRATE_...