netfilter: CLUSTERIP: clusterip_seq_stop() fix

If clusterip_seq_start() memory allocation fails, we crash later in
clusterip_seq_start(), trying to kfree(ERR_PTR(-ENOMEM))

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>