sched/nohz: Clean up nohz enter/exit

The primary observation is that nohz enter/exit is always from the
current CPU, therefore NOHZ_TICK_STOPPED does not in fact need to be
an atomic.

Secondary is that we appear to have 2 nearly identical hooks in the
nohz enter code, set_cpu_sd_state_idle() and
nohz_balance_enter_idle(). Fold the whole set_cpu_sd_state thing into
nohz_balance_{enter,exit}_idle.

Removes an atomic op from both enter and exit paths.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/time/tick-sched.c

@@ -954,13 +954,6 @@ void tick_nohz_idle_enter(void)
 	struct tick_sched *ts;
 
 	lockdep_assert_irqs_enabled();
-	/*
-	 * Update the idle state in the scheduler domain hierarchy
-	 * when tick_nohz_stop_sched_tick() is called from the idle loop.
-	 * State will be updated to busy during the first busy tick after
-	 * exiting idle.
-	 */
-	set_cpu_sd_state_idle();
 
 	local_irq_disable();