tracepoints: fix reentrancy

The tracepoints had the same problem markers did have wrt reentrancy. Apply a
similar fix using a rcu_barrier after each tracepoint mutex lock.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Signed-off-by: Ingo Molnar <mingo@elte.hu>