[NETFILTER]: Wait for untracked references in nf_conntrack module unload

Noticed by Pablo Neira <pablo@eurodev.net>.
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/netfilter/nf_conntrack_core.c

@@ -1383,6 +1383,9 @@ void nf_conntrack_cleanup(void)
 		schedule();
 		goto i_see_dead_people;
 	}
+	/* wait until all references to nf_conntrack_untracked are dropped */
+	while (atomic_read(&nf_conntrack_untracked.ct_general.use) > 1)
+		schedule();
 
 	for (i = 0; i < NF_CT_F_NUM; i++) {
 		if (nf_ct_cache[i].use == 0)