When updating/accessing the timehands, barriers are needed to ensure

When updating/accessing the timehands, barriers are needed to ensure
that:
- th_generation update is visible after the parameters update is
  visible;
- the read of parameters is not reordered before initial read of
  th_generation.

On UP kernels, compiler barriers are enough.  For SMP machines, CPU
barriers must be used too, as was confirmed by submitter by testing on
the Freescale T4240 platform with 24 PowerPC processors.

Submitted by:	Sebastian Huber <sebastian.huber@embedded-brains.de>
MFC after:	1 week