sched/eevdf: Fix RUN_TO_PARITY vs PREEMPT_SHORT

Vincent noted that RUN_TO_PARITY can prevent preemption by a shorter
slice under some conditions.

Reported-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>