drm/i915/gem: Tighten checks and acquiring the mmap object

drm/i915/gem: Tighten checks and acquiring the mmap object

Make sure we hold the rcu lock as we acquire the rcu protected reference
of the object when looking it up from the associated mmap vma.

Closes: https://gitlab.freedesktop.org/drm/intel/issues/1083
Fixes: 1798fee1aa68 ("drm/i915: Introduce DRM_I915_GEM_MMAP_OFFSET")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Abdiel Janulgue <abdiel.janulgue@linux.intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200130143931.1906301-1-chris@chris-wilson.co.uk
(cherry picked from commit 280d14a69da2e71f43408537c008f2775d5e5360)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>