BACKPORT: sched/fair: Remove spurious shorter slice preemption

Even if the waking task can preempt current, it might not be the one
selected by pick_task_fair. Check that the waking task will be selected
if we cancel the slice protection before doing so.

Bug: 438500731
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20250708165630.1948751-4-vincent.guittot@linaro.org


[Trivial conflinct in fair.c due to different code from upstream.]
(cherry picked from commit 9de74a98)
Signed-off-by: Qais Yousef <qyousef@google.com>
Change-Id: Ice4c078102e9a52b7d63d78cf6ce23874bb8180e