KVM: Fix stack-out-of-bounds read in write_mmio

[Your Real Name]

Commit: e39d200fa5bf5b94a0948db0dae44c1b73b84a56 
Target Stable Tree Branch: 4.9.y
Why this patch is needed: Due to a request to handle CVE-2017-17741, we would need to backport this patch to our kernel. The patch is already in mainline:https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=e39d200fa5bf5b94a0948db0dae44c1b73b84a56
Could you please include this patch in 4.9.y stable tree branch?
 
--AH