drm/nouveau: avoid a use-after-free when BO init fails

nouveau_bo_init() is backed by ttm_bo_init() and ferries its return code
back to the caller. On failures, ttm_bo_init() invokes the provided
destructor which should de-initialize and free the memory.

Thus, when nouveau_bo_init() returns an error the gem object has already
been released and the memory freed by nouveau_bo_del_ttm().

Fixes: 019cbd4a ("drm/nouveau: Initialize GEM object before TTM object")
Cc: Thierry Reding <treding@nvidia.com>
Signed-off-by: Jeremy Cline <jcline@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Signed-off-by: Karol Herbst <kherbst@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201203000220.18238-1-jcline@redhat.comSigned-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>

drivers/gpu/drm/nouveau/nouveau_gem.c

@@ -247,10 +247,8 @@ nouveau_gem_new(struct nouveau_cli *cli, u64 size, int align, uint32_t domain,
 	}
 
 	ret = nouveau_bo_init(nvbo, size, align, domain, NULL, NULL);
-	if (ret) {
-		nouveau_bo_ref(NULL, &nvbo);
+	if (ret)
 		return ret;
-	}
 
 	/* we restrict allowed domains on nv50+ to only the types
 	 * that were requested at creation time.  not possibly on