sched/fair: Exclude the current CPU from find_new_ilb()

It is possible for find_new_ilb() to select the current CPU, however,
this only happens from newidle balancing, in which case need_resched()
will be true, and consequently nohz_csd_func() will not trigger the
softirq.

Exclude the current CPU from becoming an ILB target.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>