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 void cfq_exit(struct io_context *ioc)
{
	rcu_read_lock();

	if (!hlist_empty(&ioc->cic_list)) {
		struct cfq_io_context *cic;

		cic = list_entry(ioc->cic_list.first, struct cfq_io_context,
								cic_list);
		cic->exit(ioc);
	}
	rcu_read_unlock();
 }

static void cfq_exit(struct io_context *ioc)
{
	rcu_read_lock();

	if (!hlist_empty(&ioc->cic_list)) {
		struct cfq_io_context *cic;

		cic = list_entry(ioc->cic_list.first, struct cfq_io_context,
								cic_list);
		cic->exit(ioc);
	}
	rcu_read_unlock();
 }
