sched_ext: Misc updates around scx_sched instance pointer

In preparation for multiple scheduler support:

- Add the @sch parameter to find_global_dsq() and refill_task_slice_dfl().

- Restructure scx_allow_ttwu_queue() and make it read scx_root into $sch.

- Make RCU protection in scx_dsq_move() and scx_bpf_dsq_move_to_local()
  explicit.

v2: Add scx_root -> sch conversion in scx_allow_ttwu_queue().

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