sched: Initialize idle tasks only once

Idle tasks are initialized via __sched_fork() twice:

     fork_idle()
        copy_process()
	  sched_fork()
             __sched_fork()
	init_idle()
          __sched_fork()

Instead of cleaning this up, sched_ext hacked around it. Even when analyis
and solution were provided in a discussion, nobody cared to clean this up.

init_idle() is also invoked from sched_init() to initialize the boot CPU's
idle task, which requires the __sched_fork() invocation. But this can be
trivially solved by invoking __sched_fork() before init_idle() in
sched_init() and removing the __sched_fork() invocation from init_idle().

Do so and clean up the comments explaining this historical leftover.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20241028103142.359584747@linutronix.de