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>
struct io_context *alloc_io_context(gfp_t gfp_flags, int node)
{
	struct io_context *ret;

	ret = kmem_cache_alloc_node(iocontext_cachep, gfp_flags, node);
	if (ret) {
		atomic_long_set(&ret->refcount, 1);
		atomic_set(&ret->nr_tasks, 1);
		spin_lock_init(&ret->lock);
		ret->ioprio_changed = 0;
		ret->ioprio = 0;
		ret->last_waited = jiffies; /* doesn't matter... */
		ret->nr_batch_requests = 0; /* because this is 0 */
		ret->aic = NULL;
		INIT_RADIX_TREE(&ret->radix_root, GFP_ATOMIC | __GFP_HIGH);
		INIT_HLIST_HEAD(&ret->cic_list);
		ret->ioc_data = NULL;
	}

	return ret;
}

struct io_context *alloc_io_context(gfp_t gfp_flags, int node)
{
	struct io_context *ret;

	ret = kmem_cache_alloc_node(iocontext_cachep, gfp_flags, node);
	if (ret) {
		atomic_long_set(&ret->refcount, 1);
		atomic_set(&ret->nr_tasks, 1);
		spin_lock_init(&ret->lock);
		ret->ioprio_changed = 0;
		ret->ioprio = 0;
		ret->last_waited = jiffies; /* doesn't matter... */
		ret->nr_batch_requests = 0; /* because this is 0 */
		ret->aic = NULL;
		INIT_RADIX_TREE(&ret->radix_root, GFP_ATOMIC | __GFP_HIGH);
		INIT_HLIST_HEAD(&ret->cic_list);
		ret->ioc_data = NULL;
	}

	return ret;
}
