kernel/sched: Fix race in k_sched_time_slice_set()

If this function is itself interrupted by a timeslice event, the
slicing state can be corrupted.  Just re-use the scheduler lock
instead of using a new spinlock; this is a low-latency function that
won't deadlock.  Found by inspection.

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