mm: compaction: do not compact pgdat for order-0

If kswapd was reclaiming for a high order and resets it to 0 due to
fragmentation it will still call compact_pgdat.  For the most part, this
will fail a compaction_suitable() test and not compact but it is
unnecessarily sloppy.  It could be fixed in the caller but fix it in the
API instead.

[dhillf@gmail.com: pointed out that it was a potential problem]
Signed-off-by: Mel Gorman <mgorman@suse.de>
Cc: Hillf Danton <dhillf@gmail.com>
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>

mm/compaction.c

@@ -1131,6 +1131,9 @@ void compact_pgdat(pg_data_t *pgdat, int order)
 		.sync = false,
 	};
 
+	if (!order)
+		return;
+
 	__compact_pgdat(pgdat, &cc);
 }