kthread: move sched-realeted initialization from kthreadd context

kthreadd is the single thread which implements ths "create" request, move
sched_setscheduler/etc from create_kthread() to kthread_create() to
improve the scalability.

We should be careful with sched_setscheduler(), use _nochek helper.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Pavel Emelyanov <xemul@openvz.org>
Cc: Vitaliy Gusev <vgusev@openvz.org
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>