[RFC v2] padata: Simplify sysfs cpumask and sequencing logic

[Wang Jinchao]

Hi, 

I've identified several potential optimizations for padata.
I'd appreciate it if you could take a look at my ideas to
see if they are feasible.

Utilizing the WQ_SYSFS from workqueue to support sysfs
======================================================

Padata relies on workqueue, and since workqueue has already implemented
support for cpumask through WQ_SYSFS, we can reuse this functionality
and avoid redundant implementation.
Link: https://docs.kernel.org/core-api/workqueue.html#affinity-scopes

Using completion to ensure the sequencing of the 'serial()'
===========================================================

In the current implementation, to ensure the sequencing of 'serial()',
we've used seq_nr, reorder_list, padata_serial_queue, reorder_work...
which has made the logic quite complex. These operations can be
simplified by using 'completion'. Specifically:
    1. in padata_do_parallel()
       1. init_completion(parallel_done) **before** queue_work
       2. queue_work(serial_work)
    2. in padata_parallel_worker
       1. complete(parallel_done) **after** parallel(padata)
    3. in padata_serial_worker
       1. wait_for_completion(parallel_done) **before** serial(padata)

Here's a simplified code snippet:

```c
struct padata_priv {
	struct completion parallel_done;
	struct work_struct	parallel_work;
	struct work_struct	serial_work;
	void   (*parallel)(struct padata_priv *padata);
	void   (*serial)(struct padata_priv *padata);
}

void padata_do_parallel(struct padata_priv *padata)
{
    ...
    init_completion(&padata->parallel_done);
	queue_work(pinst->serial_wq, &padata->serial_work);
	queue_work(pinst->parallel_wq, &padata->parallel_work);
    ...
}

static void padata_parallel_worker(struct work_struct *parallel_work)
{
	struct padata_priv *padata =
		container_of(parallel_work, struct padata_priv, parallel_work);
	padata->parallel(padata);
	// notify serial_worker to do serial()
	complete(&padata->parallel_done);
}

static void padata_serial_worker(struct work_struct *serial_work)
{
	struct padata_priv *padata =
		container_of(serial_work, struct padata_priv, serial_work);
	wait_for_completion(&padata->parallel_done);
	padata->serial(padata);
}
```