mm, page_alloc: use unlikely() in task_capc()

Hugh noted that task_capc() could use unlikely(), as most of the time
there is no capture in progress and we are in page freeing hot path.
Indeed adding unlikely() produces assembly that better matches the
assumption and moves all the tests away from the hot path.

I have also noticed that we don't need to test for cc->direct_compaction
as the only place we set current->task_capture is compact_zone_order()
which also always sets cc->direct_compaction true.

Suggested-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Hugh Dickins <hughd@googlecom>
Acked-by: Mel Gorman <mgorman@techsingularity.net>
Cc: Alex Shi <alex.shi@linux.alibaba.com>
Cc: Li Wang <liwang@redhat.com>
Link: http://lkml.kernel.org/r/4a24f7af-3aa5-6e80-4ae6-8f253b562039@suse.cz
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_alloc.c

@@ -813,11 +813,10 @@ static inline struct capture_control *task_capc(struct zone *zone)
 {
 	struct capture_control *capc = current->capture_control;
 
-	return capc &&
+	return unlikely(capc) &&
 		!(current->flags & PF_KTHREAD) &&
 		!capc->page &&
-		capc->cc->zone == zone &&
-		capc->cc->direct_compaction ? capc : NULL;
+		capc->cc->zone == zone ? capc : NULL;
 }
 
 static inline bool