f2fs: don't wake up too frequently, if there is lots of IOs

Otherwise, it wakes up discard thread which will sleep again by busy IOs
in a loop.

Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
diff --git a/fs/f2fs/segment.h b/fs/f2fs/segment.h
index a77f76f..5c7ed04 100644
--- a/fs/f2fs/segment.h
+++ b/fs/f2fs/segment.h
@@ -865,7 +865,7 @@
 		}
 	}
 	mutex_unlock(&dcc->cmd_lock);
-	if (!wakeup)
+	if (!wakeup || !is_idle(sbi, DISCARD_TIME))
 		return;
 wake_up:
 	dcc->discard_wake = 1;