powerpc/pseries: Use kmemdup

While looking at some code paths I came across this code that zeros
memory then copies over the entire length.

Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Nishanth Aravamudan <nacc@us.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>