sched: Fix faulty assertion in sched_change_end()

sched: Fix faulty assertion in sched_change_end()

Commit 47efe2ddccb1f ("sched/core: Add assertions to QUEUE_CLASS") added an
assert to sched_change_end() verifying that a class demotion would result in a
reschedule.

As it turns out; rt_mutex_setprio() does not force a resched on class
demontion. Furthermore, this is only relevant to running tasks.

Change the warning into a reschedule and make sure to only do so for running
tasks.

Fixes: 47efe2ddccb1f ("sched/core: Add assertions to QUEUE_CLASS")
Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Tested-by:  Linux Kernel Functional Testing <lkft@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20251216141725.GW3707837@noisy.programming.kicks-ass.net