sched_ext: Print sub-scheduler disabled log and reason

sched_ext: Print sub-scheduler disabled log and reason

Take scx_qmap for example, when sub scheduler is attached, there is
'BPF sub-scheduler "qmap" enabled' message, but when detached, the log
is missing. Add a new function to do the log thing, it can be used by
both root scheduler and sub scheduler.

Signed-off-by: Zhao Mengmeng <zhaomengmeng@kylinos.cn>
Reviewed-by: Cheng-Yang Chou <yphbchou0911@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>