kernel/sched: Reset time slice on swap in SMP

In uniprocessor mode, the kernel knows when a context switch "is
coming" because of the cache optimization and can use that to do
things like update time slice state.  But on SMP the scheduler state
may be updated on the other CPU at any time, so we don't know that a
switch is going to happen until the last minute.

Expose reset_time_slice() as a public function and call it when needed
out of z_swap().

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>