UBIFS: do not forget to cancel timers

This is a bug-fix: when we unmount, and we are currently in R/O
mode because of an error - we do not sync write-buffers, which
means we also do not cancel write-buffer timers we may possibly
have armed. This patch fixes the issue.

The issue can easily be reproduced by enabling UBIFS failure debug
mode (echo 4 > /sys/module/ubifs/parameters/debug_tsts) and
unmounting as soon as a failure happen. At some point the system
oopses because we have an armed hrtimer but UBIFS is unmounted
already.
Signed-off-by: NArtem Bityutskiy <Artem.Bityutskiy@nokia.com>

fs/ubifs/super.c

@@ -1772,6 +1772,10 @@ static void ubifs_put_super(struct super_block *sb)
 				 */
 				ubifs_err("failed to write master node, "
 					  "error %d", err);
+		} else {
+			for (i = 0; i < c->jhead_cnt; i++)
+				/* Make sure write-buffer timers are canceled */
+				hrtimer_cancel(&c->jheads[i].wbuf.timer);
 		}
 	}