block: Fix error path in bdrv_backing_update_filename()

error_setg_errno() takes a positive errno code. Spotted by Coverity
(CID 1381628).

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

block.c

@@ -998,7 +998,7 @@ static int bdrv_backing_update_filename(BdrvChild *c, BlockDriverState *base,
     ret = bdrv_change_backing_file(parent, filename,
                                    base->drv ? base->drv->format_name : "");
     if (ret < 0) {
-        error_setg_errno(errp, ret, "Could not update backing file link");
+        error_setg_errno(errp, -ret, "Could not update backing file link");
     }
 
     if (!(orig_flags & BDRV_O_RDWR)) {