powerpc/ftrace: show real return addresses in modules

When the function graph tracer is enabled, it replaces the return address
with a hook back to the tracer. This makes back traces see the hook instead
of the actual return address.

The current code also shows the real address by checking if the return
address jumps to the return_to_handler. If it is, is also prints out
the saved real return address.

On powerpc64, some modules may return to mod_return_to_handler, which
is not checked. This patch will also show the real address if a return
is to mod_return_to_handler as well.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>