kthread_bind: use wait_task_inactive(TASK_UNINTERRUPTIBLE)

Now that wait_task_inactive(task, state) checks task->state == state,
we can simplify the code and make this debugging check more robust.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Roland McGrath <roland@redhat.com>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>