powerpc/pseries: Read and write to the 'compressed' flag of pstore

If data returned from pstore is compressed, nvram's write callback
will add a flag ERR_TYPE_KERNEL_PANIC_GZ indicating the data is compressed
while writing to nvram. If the data read from nvram is compressed, nvram's
read callback will set the flag 'compressed'. The patch adds backward
compatibilty with old format oops header when reading from pstore.

Signed-off-by: Aruna Balakrishnaiah <aruna@linux.vnet.ibm.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Tony Luck <tony.luck@intel.com>