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