drm/i915: dma_buf_vunmap is presumed not to fail, don't let it

Since dma_buf_vunmap() procedes blithely on ignorant of whether the
driver failed to actually unmap the backing storage for the dma-buf, we
need to make a best-effort to do so. This involves not allowing
ourselves to be susceptible to signals causing us to leak the storage.

This should have been detectable with the current i-g-t as a misplaced
signal should have left the pages pinned upon freeing the object where
we have a warning in place.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem_dmabuf.c

@@ -161,12 +161,8 @@ static void i915_gem_dmabuf_vunmap(struct dma_buf *dma_buf, void *vaddr)
 {
 	struct drm_i915_gem_object *obj = dma_buf_to_obj(dma_buf);
 	struct drm_device *dev = obj->base.dev;
-	int ret;
-
-	ret = i915_mutex_lock_interruptible(dev);
-	if (ret)
-		return;
 
+	mutex_lock(&dev->struct_mutex);
 	if (--obj->vmapping_count == 0) {
 		vunmap(obj->dma_buf_vmapping);
 		obj->dma_buf_vmapping = NULL;