sched_ext: Add the @sch parameter to __bstr_format()

In preparation for multiple scheduler support, add the @sch parameter to
__bstr_format() and update the callers to read $scx_root, verify that it's
not NULL and pass it in. The passed in @sch parameter is not used yet.

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