watchdog: diag288_wdt: do not use stack buffers for hardware data

With CONFIG_VMAP_STACK=y the stack is allocated from the vmalloc space.
Data passed to a hardware or a hypervisor interface that
requires V=R can no longer be allocated on the stack.

Use kmalloc() to get memory for a diag288 command.

Signed-off-by: Alexander Egorenkov <egorenar@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>