drm/i915: Check obj->vma_list under the struct_mutex

When we walk the list of vma, or even for protecting against concurrent
framebuffer creation, we must hold the struct_mutex or else a second
thread can corrupt the list as we walk it.

Fixes regression from
commit d7f46fc
Author: Ben Widawsky <benjamin.widawsky@intel.com>
Date:   Fri Dec 6 14:10:55 2013 -0800

    drm/i915: Make pin count per VMA

References: https://bugs.freedesktop.org/show_bug.cgi?id=89085
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: stable@vger.kernel.org
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/i915_gem_tiling.c

@@ -335,9 +335,10 @@ i915_gem_set_tiling(struct drm_device *dev, void *data,
 		return -EINVAL;
 	}
 
+	mutex_lock(&dev->struct_mutex);
 	if (i915_gem_obj_is_pinned(obj) || obj->framebuffer_references) {
-		drm_gem_object_unreference_unlocked(&obj->base);
-		return -EBUSY;
+		ret = -EBUSY;
+		goto err;
 	}
 
 	if (args->tiling_mode == I915_TILING_NONE) {
@@ -369,7 +370,6 @@ i915_gem_set_tiling(struct drm_device *dev, void *data,
 		}
 	}
 
-	mutex_lock(&dev->struct_mutex);
 	if (args->tiling_mode != obj->tiling_mode ||
 	    args->stride != obj->stride) {
 		/* We need to rebind the object if its current allocation
@@ -424,6 +424,7 @@ i915_gem_set_tiling(struct drm_device *dev, void *data,
 		obj->bit_17 = NULL;
 	}
 
+err:
 	drm_gem_object_unreference(&obj->base);
 	mutex_unlock(&dev->struct_mutex);