tracepoints: tracepoint_synchronize_unregister()

Create tracepoint_synchronize_unregister() which must be called 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: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Signed-off-by: Ingo Molnar <mingo@elte.hu>