Impact: fix a bug in sample tracepoints

Need a tracepoint_synchronize_unregister() before the end of exit() to
make sure every probe callers have exited the non preemptible section
and thus are not executing the probe code anymore.
Signed-off-by: NMathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Signed-off-by: NIngo Molnar <mingo@elte.hu>