kernel/timeout: Fix race with clock timeout setting

The call to z_clock_set_timeout() was being made outside the timeout
lock, which can race against other contexts setting sooner-expiring
timeouts.

Also add a long comment to one spot (timeslicing) where this call is
made outside the timeout spinlock (inside the scheduler lock) and why
this is OK.

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