UBIFS: slightly optimize write-buffer timer usage

This patch adds the following minor optimization:

1. If write-buffer does not use the timer, indicate it with the
   wbuf->no_timer variable, instead of using the wbuf->softlimit
   variable. This is better because wbuf->softlimit is of ktime_t
   type, and the ktime_to_ns function contains 64-bit multiplication.

2. Do not call the 'hrtimer_cancel()' function for write-buffers
   which do not use timers.

3. Do not cancel the timer in 'ubifs_put_super()' because the
   synchronization function does this.

This patch also removes a confusing comment.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>

fs/ubifs/io.c

@@ -312,7 +312,7 @@ static void new_wbuf_timer_nolock(struct ubifs_wbuf *wbuf)
 {
 	ubifs_assert(!hrtimer_active(&wbuf->timer));
 
-	if (!ktime_to_ns(wbuf->softlimit))
+	if (wbuf->no_timer)
 		return;
 	dbg_io("set timer for jhead %d, %llu-%llu millisecs", wbuf->jhead,
 	       ktime_to_ns(wbuf->softlimit)/USEC_PER_SEC,
@@ -327,11 +327,8 @@ static void new_wbuf_timer_nolock(struct ubifs_wbuf *wbuf)
  */
 static void cancel_wbuf_timer_nolock(struct ubifs_wbuf *wbuf)
 {
-	/*
-	 * If the syncer is waiting for the lock (from the background thread's
-	 * context) and another task is changing write-buffer then the syncing
-	 * should be canceled.
-	 */
+	if (wbuf->no_timer)
+		return;
 	wbuf->need_sync = 0;
 	hrtimer_cancel(&wbuf->timer);
 }