sched: Account PREEMPT_ACTIVE context as atomic

PREEMPT_ACTIVE implies non-preemptible context and thus atomic context
despite what in_atomic*() APIs reports about it. These functions
shouldn't ignore this value like they are currently doing.

It appears that these APIs were ignoring PREEMPT_ACTIVE in order to
ease the check in schedule_debug(). Meanwhile it is sufficient to rely
on PREEMPT_ACTIVE in order to disable preemption in __schedule().

So lets fix the in_atomic*() APIs and simplify the preempt count ops
on __schedule() callers.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>