tracing: Add rcu dereference annotation for test func that touches filter->prog

A boot up test function update_pred_fn() dereferences filter->prog without
the proper rcu annotation.

To do this, we must also take the event_mutex first. Normally, this isn't
needed because this test function can not race with other use cases that
touch the event filters (it is disabled if any events are enabled).

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Fixes: 80765597 ("tracing: Rewrite filter logic to be simpler and faster")
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>