powerpc/pseries: Add backward compatibilty to read old kernel oops-log

Older kernels has just length information in their header. Handle it
while reading old kernel oops log from pstore.

Applies on top of powerpc/pseries: Fix buffer overflow when reading from pstore

Signed-off-by: Aruna Balakrishnaiah <aruna@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>