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