sched: Switch rq->next_class in proxy_reset_donor()

Similar to commit 04f80f8b12a0 ("sched: Switch rq->next_class on
proxy_resched_idle()"), we should also set the rq->next_class
when we call proxy_reset_donor().

Fixes: f13beb010e4a ("sched: Have try_to_wake_up() handle return-migration for PROXY_WAKING case")
Signed-off-by: John Stultz <jstultz@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260807035232.1881495-6-jstultz@google.com