Patch from Marc Singer

As suggested by Russell, we add a barrier() before returning from
stack_trace().  This was helpful when diagnosing a problem with a
kernel transition to user-space where the problem was a lack of
floating point support in the kernel.  Without this change, the
backtrace reported an error.

It is possible that this change has already been made.  I don't see it
in any of the applied patches that I can read.