netfilter: ebt_ulog: remove unnecessary spin lock protection

No need for spinlock to protect the netlink skb in the
ebt_ulog_fini path. We are sure there is noone using it
at that stage.

Signed-off-by: Gao feng <gaofeng@cn.fujitsu.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>