workqueue: Break up enum definitions and give names to the types

workqueue: Break up enum definitions and give names to the types

workqueue is collecting different sorts of enums into a single unnamed enum
type which can increase confusion around enum width. Also, unnamed enums
can't be accessed from BPF. Let's break up enum definitions according to
their purposes and give them type names.

Signed-off-by: Tejun Heo <tj@kernel.org>