sched_ext: Allow forcibly picking an scx task

sched_ext: Allow forcibly picking an scx task

Refactor pick_task_scx() adding a new argument to forcibly pick a
SCHED_EXT task, ignoring any higher-priority sched class activity.

This refactoring prepares the code for future scenarios, e.g., allowing
the ext dl_server to force a SCHED_EXT task selection.

No functional changes.

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