drm/client: Fix error code in drm_client_buffer_vmap_local()

This function accidentally returns zero/success on the failure path.
It leads to locking issues and an uninitialized *map_copy in the
caller.

Fixes: b4b0193e ("drm/fbdev-generic: Fix locking with drm_client_buffer_vmap_local()")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/89d13df3-747c-4c5d-b122-d081aef5110a@stanley.mountain

drivers/gpu/drm/drm_client.c

@@ -355,7 +355,7 @@ int drm_client_buffer_vmap_local(struct drm_client_buffer *buffer,
 
 err_drm_gem_vmap_unlocked:
 	drm_gem_unlock(gem);
-	return 0;
+	return ret;
 }
 EXPORT_SYMBOL(drm_client_buffer_vmap_local);