[NETFILTER] fix leak of fragment queue at unloading nf_conntrack_ipv6

This patch makes nf_conntrack_ipv6 free all IPv6 fragment queues at module
unloading time.  Also introduce a BUG_ON if we ever again have leaks in
the memory accounting.
Signed-off-by: NYasuyuki Kozakai <yasuyuki.kozakai@toshiba.co.jp>
Signed-off-by: NHarald Welte <laforge@netfilter.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/netfilter/nf_conntrack_reasm.c

@@ -282,6 +282,7 @@ static void nf_ct_frag6_evictor(void)
 			return;
 		}
 		tmp = nf_ct_frag6_lru_list.next;
+		BUG_ON(tmp == NULL);
 		fq = list_entry(tmp, struct nf_ct_frag6_queue, lru_list);
 		atomic_inc(&fq->refcnt);
 		read_unlock(&nf_ct_frag6_lock);
@@ -891,5 +892,6 @@ int nf_ct_frag6_init(void)
 void nf_ct_frag6_cleanup(void)
 {
 	del_timer(&nf_ct_frag6_secret_timer);
+	nf_ct_frag6_low_thresh = 0;
 	nf_ct_frag6_evictor();
 }