powerpc/perf/imc: Use cpu_to_node() not topology_physical_package_id()

init_imc_pmu() uses topology_physical_package_id() to detect the
node id of the processor it is on to get local memory, but that's
wrong, and can lead to crashes. Fix it to use cpu_to_node().

Fixes: 885dcd70

 ("powerpc/perf: Add nest IMC PMU support")
Cc: stable@vger.kernel.org # v4.14+
Reported-By: Rob Lippert <rlippert@google.com>
Tested-By: Madhavan Srinivasan <maddy@linux.vnet.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>