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: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/segment.h

@@ -865,7 +865,7 @@ static inline void wake_up_discard_thread(struct f2fs_sb_info *sbi, bool force)
 		}
 	}
 	mutex_unlock(&dcc->cmd_lock);
-	if (!wakeup)
+	if (!wakeup || !is_idle(sbi, DISCARD_TIME))
 		return;
 wake_up:
 	dcc->discard_wake = 1;