tracepoint: Remove smp_read_barrier_depends() from comment

The comment in tracepoint_add_func() mentions smp_read_barrier_depends(),
whose use should be quite restricted.  This commit updates the comment
to instead mention the smp_store_release() and rcu_dereference_sched()
that the current code actually uses.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@kernel.org>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>