sched_ext: Separate out scx_kick_cpu() and add @sch to it

In preparation for multiple scheduler support, separate out scx_kick_cpu()
from scx_bpf_kick_cpu() and add the @sch parameter to it. scx_bpf_kick_cpu()
now acquires an RCU read lock, reads $scx_root, and calls scx_kick_cpu()
with it if non-NULL. The passed in @sch parameter is not used yet.

Internal uses of scx_bpf_kick_cpu() are converted to scx_kick_cpu(). Where
$sch is available, it's used. In the pick_task_scx() path where no
associated scheduler can be identified, $scx_root is used directly. Note
that $scx_root cannot be NULL in this case.

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