sched: Fix integer overflow in sched_update_nr_prod()

"int" type is used to hold the time difference between the successive
updates to nr_run in sched_update_nr_prod(). This can result in
overflow, if the function is called ~2.15 sec after it was called
before. The most probable scenarios are when CPU is idle and
hotplugged. But as we update the last_time of all possible CPUs in
sched_get_nr_running_avg() periodically from a deferrable timer context
(core_ctl module), this overflow is observed only when the system is
completely idle for long time. When this overflow happens we hit
a BUG_ON() in sched_get_nr_running_avg().

Use "u64" type instead of "int" for holding the time difference and
add additional BUG_ON() to catch the instances where sched_clock()
returns a backward value.

Change-Id: I284abb5889ceb8cf9cc689c79ed69422a0e74986
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>
Signed-off-by: Albert I <krascgq@outlook.co.id>