[PATCH v5] erofs: add per-cpu threads for decompression as an option

[Sandeep Dhavale]

Hi all,
I completed the tests and the results are consistent with
our previous observation. We can see that removing WQ_UNBOUND
helps but the scheduling latency by using high priority per cpu
kthreads is even lower. Below is the table.

|---------------------+-------+-------+------+-------|
| Table               | avg   | med   | min  | max   |
|---------------------+-------+-------+------+-------|
| Default erofs       | 19323 | 19758 | 3986 | 35051 |
|---------------------+-------+-------+------+-------|
| !WQ_UNBOUND         | 11202 | 10798 | 3493 | 19822 |
|---------------------+-------+-------+------+-------|
| hipri pcpu kthreads | 7182  | 7017  | 2463 | 12300 |
|---------------------+-------+-------+------+-------|


Thanks,
Sandeep.