sched, sched_ext: Move some declarations from kernel/sched/ext.h to sched.h

While sched_ext was out of tree, everything sched_ext specific which can be
put in kernel/sched/ext.h was put there to ease forward porting. However,
kernel/sched/sched.h is the better location for some of them. Relocate.

- struct sched_enq_and_set_ctx, sched_deq_and_put_task() and
  sched_enq_and_set_task().

- scx_enabled() and scx_switched_all().

- for_active_class_range() and for_each_active_class(). sched_class
  declarations are moved above the class iterators for this.

No functional changes intended.

Signed-off-by: Tejun Heo <tj@kernel.org>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: David Vernet <void@manifault.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>