drm/i915/dmabuf: Tighten struct_mutex for unmap_dma_buf

We only need the struct_mutex to manipulate the pages_pin_count on the
object, we do not need to hold our BKL when freeing the exported
scatterlist.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1460113874-17366-2-git-send-email-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem_dmabuf.c

@@ -95,14 +95,12 @@ static void i915_gem_unmap_dma_buf(struct dma_buf_attachment *attachment,
 {
 	struct drm_i915_gem_object *obj = dma_buf_to_obj(attachment->dmabuf);
 
-	mutex_lock(&obj->base.dev->struct_mutex);
-
 	dma_unmap_sg(attachment->dev, sg->sgl, sg->nents, dir);
 	sg_free_table(sg);
 	kfree(sg);
 
+	mutex_lock(&obj->base.dev->struct_mutex);
 	i915_gem_object_unpin_pages(obj);
-
 	mutex_unlock(&obj->base.dev->struct_mutex);
 }