drm/i915/ttm: add some sanity checks for lmem_userfault_list

Rather than getting some hard to debug uaf, add some warns to hopefully
catch issues with userfault_count being non-zero when destroying the
object. Also if we somehow add an object to lmem_userfault_list that
somehow doesn't map lmem.

References: https://gitlab.freedesktop.org/drm/intel/-/issues/7469Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Anshuman Gupta <anshuman.gupta@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Andrzej Hajda <andrzej.hajda@intel.com>
Cc: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221107165414.56970-2-matthew.auld@intel.com

drivers/gpu/drm/i915/gem/i915_gem_ttm.c

@@ -1098,6 +1098,8 @@ static vm_fault_t vm_fault_ttm(struct vm_fault *vmf)
 		spin_lock(&to_i915(obj->base.dev)->runtime_pm.lmem_userfault_lock);
 		list_add(&obj->userfault_link, &to_i915(obj->base.dev)->runtime_pm.lmem_userfault_list);
 		spin_unlock(&to_i915(obj->base.dev)->runtime_pm.lmem_userfault_lock);
+
+		GEM_WARN_ON(!i915_ttm_cpu_maps_iomem(bo->resource));
 	}
 
 	if (wakeref & CONFIG_DRM_I915_USERFAULT_AUTOSUSPEND)
@@ -1180,6 +1182,8 @@ static void i915_ttm_unmap_virtual(struct drm_i915_gem_object *obj)
 		}
 	}
 
+	GEM_WARN_ON(obj->userfault_count);
+
 	ttm_bo_unmap_virtual(i915_gem_to_ttm(obj));
 
 	if (wakeref)