sched: Add BUG_ON when task_cpu() is incorrect

It would be fatal if task_cpu() information for a task does not
accurately represent the cpu on which its running. All sorts of wierd
issues can arise if that were to happen! Add a BUG_ON() in context
switch to detect such cases.

Change-Id: I4eb2c96c850e2247e22f773bbb6eedb8ccafa49c
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>