mm, compaction: cover all compaction mode in compact_zone

The compiler is complaining after "mm, compaction: change COMPACT_
constants into enum"

  mm/compaction.c: In function `compact_zone':
  mm/compaction.c:1350:2: warning: enumeration value `COMPACT_DEFERRED' not handled in switch [-Wswitch]
    switch (ret) {
    ^
  mm/compaction.c:1350:2: warning: enumeration value `COMPACT_COMPLETE' not handled in switch [-Wswitch]
  mm/compaction.c:1350:2: warning: enumeration value `COMPACT_NO_SUITABLE_PAGE' not handled in switch [-Wswitch]
  mm/compaction.c:1350:2: warning: enumeration value `COMPACT_NOT_SUITABLE_ZONE' not handled in switch [-Wswitch]
  mm/compaction.c:1350:2: warning: enumeration value `COMPACT_CONTENDED' not handled in switch [-Wswitch]

compaction_suitable is allowed to return only COMPACT_PARTIAL,
COMPACT_SKIPPED and COMPACT_CONTINUE so other cases are simply
impossible.  Put a VM_BUG_ON to catch an impossible return value.
Signed-off-by: Michal Hocko <mhocko@suse.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Hillf Danton <hillf.zj@alibaba-inc.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Joonsoo Kim <js1304@gmail.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Vladimir Davydov <vdavydov@virtuozzo.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/compaction.c

@@ -1383,15 +1383,12 @@ static enum compact_result compact_zone(struct zone *zone, struct compact_contro
 
 	ret = compaction_suitable(zone, cc->order, cc->alloc_flags,
 							cc->classzone_idx);
-	switch (ret) {
-	case COMPACT_PARTIAL:
-	case COMPACT_SKIPPED:
-		/* Compaction is likely to fail */
+	/* Compaction is likely to fail */
+	if (ret == COMPACT_PARTIAL || ret == COMPACT_SKIPPED)
 		return ret;
-	case COMPACT_CONTINUE:
-		/* Fall through to compaction */
-		;
-	}
+
+	/* huh, compaction_suitable is returning something unexpected */
+	VM_BUG_ON(ret != COMPACT_CONTINUE);
 
 	/*
 	 * Clear pageblock skip if there were failures recently and compaction