powerpc/pseries: Fix oops in hotplug memory notifier

During post-migration device tree updates, we can oops in
pseries_update_drconf_memory() if the source device tree has an
ibm,dynamic-memory-v2 property and the destination has a
ibm,dynamic_memory (v1) property. The notifier processes an "update"
for the ibm,dynamic-memory property but it's really an add in this
scenario. So make sure the old property object is there before
dereferencing it.

Fixes: 2b31e3ae ("powerpc/drmem: Add support for ibm, dynamic-memory-v2 property")
Cc: stable@vger.kernel.org # v4.16+
Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>