sched: Fix affine_move_task() self-concurrency

Consider:

   sched_setaffinity(p, X);		sched_setaffinity(p, Y);

Then the first will install p->migration_pending = &my_pending; and
issue stop_one_cpu_nowait(pending); and the second one will read
p->migration_pending and _also_ issue: stop_one_cpu_nowait(pending),
the _SAME_ @pending.

This causes stopper list corruption.

Add set_affinity_pending::stop_pending, to indicate if a stopper is in
progress.

Fixes: 6d337eab

 ("sched: Fix migrate_disable() vs set_cpus_allowed_ptr()")
Cc: stable@kernel.org
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Link: https://lkml.kernel.org/r/20210224131355.649146419@infradead.org