async: bring sanity to the use of words domain and running

async: bring sanity to the use of words domain and running

In the beginning, running lists were literal struct list_heads.  Later
on, struct async_domain was added.  For some reason, while the
conversion substituted list_heads with async_domains, the variable
names weren't fully converted.  In more places, "running" was used for
struct async_domain while other places adopted new "domain" name.

The situation is made much worse by having async_domain's running list
named "domain" and async_entry's field pointing to async_domain named
"running".

So, we end up with mix of "running" and "domain" for variable names
for async_domain, with the field names of async_domain and async_entry
swapped between "running" and "domain".

It feels almost intentionally made to be as confusing as possible.
Bring some sanity by

* Renaming all async_domain variables "domain".

* s/async_running/async_dfl_domain/

* s/async_domain->domain/async_domain->running/

* s/async_entry->running/async_entry->domain/

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Dan Williams <djbw@fb.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>