[drivers/virt/fsl_hypervisor.c: prevent integer overflow in ioctl]

It was discovered that an integer overflow existed in the Freescale
(PowerPC) hypervisor manager in the Linux kernel. A local attacker
with write access to /dev/fsl-hv could use this to cause a denial
of service (system crash) or possibly execute arbitrary code.

 sbeattie> depends on freescale (ppc) only
 tyhicks> Only the powerpc-e500mc and powerpc64-emb flavours of the Xenial
  powerpc kernel are affected. I'm downgrading the priority to negligible since
  no officially supported architectures are affected.