drm/i915/gvt: Fix mmap range check

drm/i915/gvt: Fix mmap range check

This is to fix missed mmap range check on vGPU bar2 region
and only allow to map vGPU allocated GMADDR range, which means
user space should support sparse mmap to get proper offset for
mmap vGPU aperture. And this takes care of actual pgoff in mmap
request as original code always does from beginning of vGPU
aperture.

Fixes: 659643f7d814 ("drm/i915/gvt/kvmgt: add vfio/mdev support to KVMGT")
Cc: "Monroy, Rodrigo Axel" <rodrigo.axel.monroy@intel.com>
Cc: "Orrala Contreras, Alfredo" <alfredo.orrala.contreras@intel.com>
Cc: stable@vger.kernel.org # v4.10+
Reviewed-by: Hang Yuan <hang.yuan@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
CVE-2019-11085

(cherry picked from commit 51b00d8509dc69c98740da2ad07308b630d3eb7d)
Signed-off-by: Timo Aaltonen <timo.aaltonen@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Acked-by: Tyler Hicks <tyhicks@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>