sched/debug: Provide slice length for fair tasks

Since commit:

857b158dc5e8 ("sched/eevdf: Use sched_attr::sched_runtime to set request/slice suggestion")

... we have the userspace per-task tunable slice length, which is
a key parameter that is otherwise difficult to obtain, so provide
it in /proc/$PID/sched.

[ mingo: Clarified the changelog. ]

Signed-off-by: Christian Loehle <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

kernel/sched/debug.c

···
 	if (task_has_dl_policy(p)) {
 		P(dl.runtime);
 		P(dl.deadline);
+	} else if (fair_policy(p->policy)) {
+		P(se.slice);
 	}
 #ifdef CONFIG_SCHED_CLASS_EXT
 	__PS("ext.enabled", task_on_scx(p));