futex: use current->time_slack_ns for rt tasks too

RT tasks should set their timer slack to 0 on their own.  This
patch removes the 'if (rt_task()) slack = 0;' block in
futex_wait.

Build and boot tested on a 4 way Intel x86_64 workstation.
Passes basic pthread_mutex and PI tests out of
ltp/testcases/realtime.
Signed-off-by: Darren Hart <dvhltc@us.ibm.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Arjan van de Ven <arjan@linux.intel.com>
LKML-Reference: <20090312075559.9856.28822.stgit@Aeon>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/futex.c

@@ -1253,16 +1253,13 @@ static int futex_wait(u32 __user *uaddr, int fshared,
 		if (!abs_time)
 			schedule();
 		else {
-			unsigned long slack;
-			slack = current->timer_slack_ns;
-			if (rt_task(current))
-				slack = 0;
 			hrtimer_init_on_stack(&t.timer,
 					      clockrt ? CLOCK_REALTIME :
 					      CLOCK_MONOTONIC,
 					      HRTIMER_MODE_ABS);
 			hrtimer_init_sleeper(&t, current);
-			hrtimer_set_expires_range_ns(&t.timer, *abs_time, slack);
+			hrtimer_set_expires_range_ns(&t.timer, *abs_time,
+						     current->timer_slack_ns);
 
 			hrtimer_start_expires(&t.timer, HRTIMER_MODE_ABS);
 			if (!hrtimer_active(&t.timer))