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>

include/linux/sched/nohz.h

@@ -16,11 +16,9 @@ static inline void cpu_load_update_nohz_stop(void) { }
 
 #if defined(CONFIG_SMP) && defined(CONFIG_NO_HZ_COMMON)
 extern void nohz_balance_enter_idle(int cpu);
-extern void set_cpu_sd_state_idle(void);
 extern int get_nohz_timer_target(void);
 #else
 static inline void nohz_balance_enter_idle(int cpu) { }
-static inline void set_cpu_sd_state_idle(void) { }
 #endif
 
 #ifdef CONFIG_NO_HZ_COMMON