sched: Use __always_inline on is_idle_task()

is_idle_task() may be used from noinstr functions such as
irqentry_enter(). Since the compiler is free to not inline regular
inline functions, switch to using __always_inline.
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200820172046.GA177701@elver.google.com

include/linux/sched.h

@@ -1666,7 +1666,7 @@ extern struct task_struct *idle_task(int cpu);
  *
  * Return: 1 if @p is an idle task. 0 otherwise.
  */
-static inline bool is_idle_task(const struct task_struct *p)
+static __always_inline bool is_idle_task(const struct task_struct *p)
 {
 	return !!(p->flags & PF_IDLE);
 }