sched_ext: Refactor task init/exit helpers

sched_ext: Refactor task init/exit helpers

- Add the @sch parameter to scx_init_task() and drop @tg as it can be
  obtained from @p. Separate out __scx_init_task() which does everything
  except for the task state transition.

- Add the @sch parameter to scx_enable_task(). Separate out
  __scx_enable_task() which does everything except for the task state
  transition.

- Add the @sch parameter to scx_disable_task().

- Rename scx_exit_task() to scx_disable_and_exit_task() and separate out
  __scx_disable_and_exit_task() which does everything except for the task
  state transition.

While some task state transitions are relocated, no meaningful behavior
changes are expected.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>