[RFC/Feature Request] bcache: support io priority

Hi bcache developers,

Many business scenarios require different IO priorities. After setting IO priorities, the goal is to have high-priority writes go to the SSD cache, while low-priority requests are bypassed to the HDD. Currently, the logic is:
```

if ((bio->bi_opf & (REQ_RAHEAD|REQ_BACKGROUND))) {

if (!(bio->bi_opf & (REQ_META|REQ_PRIO)) &&

(dc->cache_readahead_policy != BCH_CACHE_READA_ALL))

goto skip;

} 
```

This logic allows IO requests to bypass, but the request type cannot be controlled in user space.

Could you support setting bypass based on IO request priority? Is such a requirement reasonable?