Fix potential endless loop in kswapd when compaction is not enabled

We should only test compaction_suitable if the kernel is built with
CONFIG_COMPACTION, otherwise the stub compaction_suitable function will
always return COMPACT_SKIPPED and send kswapd into an infinite loop.

Reported-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/vmscan.c

@@ -2946,7 +2946,8 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
 				continue;
 
 			/* Would compaction fail due to lack of free memory? */
-			if (compaction_suitable(zone, order) == COMPACT_SKIPPED)
+			if (COMPACTION_BUILD &&
+			    compaction_suitable(zone, order) == COMPACT_SKIPPED)
 				goto loop_again;
 
 			/* Confirm the zone is balanced for order-0 */