io_uring: cleanup handling of the two task_work lists

Rather than pass in a bool for whether or not this work item needs to go
into the priority list or not, provide separate helpers for it. For most
use cases, this also then gets rid of the branch for non-priority task
work.

While at it, rename the prior_task_list to prio_task_list. Prior is
a confusing name for it, as it would seem to indicate that this is the
previous task_work list. prio makes it clear that this is a priority
task_work list.

Signed-off-by: Jens Axboe <axboe@kernel.dk>