mm: vmscan: rework compaction-ready signaling in direct reclaim

Page reclaim for a higher-order page runs until compaction is ready,
then aborts and signals this situation through the return value of
shrink_zones().  This is an oddly specific signal to encode in the
return value of shrink_zones(), though, and can be quite confusing.

Introduce sc->compaction_ready and signal the compactability of the
zones out-of-band to free up the return value of shrink_zones() for
actual zone reclaimability.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Rik van Riel <riel@redhat.com>
Cc: Michal Hocko <mhocko@suse.cz>
Acked-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>