powerpc/powernv: Check sysfs size before copying

The sysparam code currently uses the userspace supplied number of
bytes when memcpy()ing in to a local 64-byte buffer.

Limit the maximum number of bytes by the size of the buffer.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

diff --cc arch/powerpc/platforms/powernv/opal-sysparam.c
Simple merge