sched/core: Ensure cpu number is valid

Sched_cpu_isolate takes cpu number as input. The input is not
being checked against the NR_CPUS which could potentially
overflow the buffer cpu_isolation_vote. Fix it by checking the
boundary.

Change-Id: If5e07b2481454b539ccdca15a302722478d22df4
Signed-off-by: Runmin Wang <runminw@codeaurora.org>