writeback: do uninterruptible sleep in balance_dirty_pages()

Using TASK_INTERRUPTIBLE in balance_dirty_pages() seems wrong.  If it's
going to do that then it must break out if signal_pending(), otherwise
it's pretty much guaranteed to degenerate into a busywait loop.  Plus we
*do* want these processes to appear in D state and to contribute to load
average.

So it should be TASK_UNINTERRUPTIBLE.                 -- Andrew Morton

Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page-writeback.c

@@ -563,7 +563,7 @@ static void balance_dirty_pages(struct address_space *mapping,
 				break;		/* We've done our duty */
 		}
 		trace_wbc_balance_dirty_wait(&wbc, bdi);
-		__set_current_state(TASK_INTERRUPTIBLE);
+		__set_current_state(TASK_UNINTERRUPTIBLE);
 		io_schedule_timeout(pause);
 
 		/*