powerpc/ftrace: bugfix for test_24bit_addr

The branch target should be the func addr, not the addr of func_descr_t.
So using ppc_function_entry() to generate the right target addr.

Signed-off-by: Liu Ping Fan <pingfank@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>