block: Fix io_context leak after failure of clone with CLONE_IO

With CLONE_IO, parent's io_context->nr_tasks is incremented, but never
decremented whenever copy_process() fails afterwards, which prevents
exit_io_context() from calling IO schedulers exit functions.

Give a task_struct to exit_io_context(), and call exit_io_context() instead of
put_io_context() in copy_process() cleanup path.

Signed-off-by: Louis Rilling <louis.rilling@kerlabs.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
static int do_wait_thread(struct wait_opts *wo, struct task_struct *tsk)
{
	struct task_struct *p;

	list_for_each_entry(p, &tsk->children, sibling) {
		/*
		 * Do not consider detached threads.
		 */
		if (!task_detached(p)) {
			int ret = wait_consider_task(wo, 0, p);
			if (ret)
				return ret;
		}
	}

	return 0;
}

static int do_wait_thread(struct wait_opts *wo, struct task_struct *tsk)
{
	struct task_struct *p;

	list_for_each_entry(p, &tsk->children, sibling) {
		/*
		 * Do not consider detached threads.
		 */
		if (!task_detached(p)) {
			int ret = wait_consider_task(wo, 0, p);
			if (ret)
				return ret;
		}
	}

	return 0;
}
