ppc/pnv: check size before data buffer access

ppc/pnv: check size before data buffer access

While performing PowerNV memory r/w operations, the access length
'sz' could exceed the data[4] buffer size. Add check to avoid OOB
access.

Reported-by: Moguofang <moguofang@huawei.com>
Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
(cherry picked from commit d07945e78eb6b593cd17a4640c1fc9eb35e3245d)
*CVE-2018-18954
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>