netfilter: nf_tables: Fix an Oops in nf_tables_updobj() error handling

The "newobj" is an error pointer so we can't pass it to kfree().  It
doesn't need to be freed so we can remove that and I also renamed the
error label.

Fixes: d62d0ba9

 ("netfilter: nf_tables: Introduce stateful object update operation")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Fernando Fernandez Mancera <ffmancera@riseup.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>