kthread: Include kthreadd to the managed affinity list

kthread: Include kthreadd to the managed affinity list

The unbound kthreads affinity management performed by cpuset is going to
be imported to the kthread core code for consolidation purposes.

Treat kthreadd just like any other kthread.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Reviewed-by: Waiman Long <longman@redhat.com>
Cc: Marco Crivellari <marco.crivellari@suse.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Waiman Long <longman@redhat.com>