I quickly went through prototyping something, only to detail the RFC (so
it's not really tested etc...)
Also, it's not something that I am planning to send right away as it
breaks the former API.
For the user data pointer, it's only a matter to use CONTAINER_OF() with
given cb pointer
as Johan noticed.
I put the generic callback list functions into a header in
drivers/gpio/, not sure it's would be the final place.