tracing: Add rcu dereference annotation for filter->prog

ftrace_function_set_filter() referenences filter->prog without annotation
and sparse complains about it. It needs a rcu_dereference_protected()
wrapper.

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>