sched: Allow pinned user tasks to be awakened to the CPU they pinned

Since commit 7af443ee

 ("sched/core: Require cpu_active() in
select_task_rq(), for user tasks") select_fallback_rq() will BUG() if
the CPU to which a task has pinned itself and pinned becomes
!cpu_active() while it slept.
The task will continue running on the to-be-removed CPU and will remove
itself from the CPU during takedown_cpu() (while cpuhp_pin_lock will be
acquired) and move to another CPU based on its mask after the
migrate_disable() section has been left.

Cc: stable-rt@vger.kernel.org
Signed-off-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>