drm: Fix error handling in drm_legacy_addctx

'ctx->handle' is unsigned, it never less than zero.
This patch use int 'tmp_handle' to handle the err condition.

Fixes: 6296814 ("drm: convert drm context code to use Linux idr")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20181229024907.12852-1-yuehaibing@huawei.com

drivers/gpu/drm/drm_context.c

@@ -361,23 +361,26 @@ int drm_legacy_addctx(struct drm_device *dev, void *data,
 {
 	struct drm_ctx_list *ctx_entry;
 	struct drm_ctx *ctx = data;
+	int tmp_handle;
 
 	if (!drm_core_check_feature(dev, DRIVER_KMS_LEGACY_CONTEXT) &&
 	    !drm_core_check_feature(dev, DRIVER_LEGACY))
 		return -EOPNOTSUPP;
 
-	ctx->handle = drm_legacy_ctxbitmap_next(dev);
-	if (ctx->handle == DRM_KERNEL_CONTEXT) {
+	tmp_handle = drm_legacy_ctxbitmap_next(dev);
+	if (tmp_handle == DRM_KERNEL_CONTEXT) {
 		/* Skip kernel's context and get a new one. */
-		ctx->handle = drm_legacy_ctxbitmap_next(dev);
+		tmp_handle = drm_legacy_ctxbitmap_next(dev);
 	}
-	DRM_DEBUG("%d\n", ctx->handle);
-	if (ctx->handle < 0) {
+	DRM_DEBUG("%d\n", tmp_handle);
+	if (tmp_handle < 0) {
 		DRM_DEBUG("Not enough free contexts.\n");
 		/* Should this return -EBUSY instead? */
-		return -ENOMEM;
+		return tmp_handle;
 	}
 
+	ctx->handle = tmp_handle;
+
 	ctx_entry = kmalloc(sizeof(*ctx_entry), GFP_KERNEL);
 	if (!ctx_entry) {
 		DRM_DEBUG("out of memory\n");