sched/fair: Rename variable cpu_util eff_util

cppcheck reports
kernel/sched/fair.c:7436:17: style: Local variable 'cpu_util' shadows outer function [shadowFunction]
  unsigned long cpu_util;
                ^

Clean this up by renaming the variable to eff_util

Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Valentin Schneider <vschneid@redhat.com>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Link: https://lore.kernel.org/r/20230611122535.183654-1-trix@redhat.com