fprobe: Fix sparse warning for acccessing __rcu ftrace_hash

Since ftrace_ops::local_hash::filter_hash field is an __rcu pointer,
we have to use rcu_access_pointer() to access it.
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/164802093635.1732982.4938094876018890866.stgit@devnote2

kernel/trace/fprobe.c

@@ -215,7 +215,7 @@ int register_fprobe(struct fprobe *fp, const char *filter, const char *notfilter
 	 * correctly calculate the total number of filtered symbols
 	 * from both filter and notfilter.
 	 */
-	hash = fp->ops.local_hash.filter_hash;
+	hash = rcu_access_pointer(fp->ops.local_hash.filter_hash);
 	if (WARN_ON_ONCE(!hash))
 		goto out;