sched/fair: Move call to list_last_entry() in detach_tasks

Move the call to list_last_entry() in detach_tasks() after testing
loop_max and loop_break.

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20220825122726.20819-4-vincent.guittot@linaro.org