[NETFILTER]: Wait until all references to ip_conntrack_untracked are dropped on unload

Fixes a crash when unloading ip_conntrack.
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/netfilter/ip_conntrack_core.c

@@ -1107,6 +1107,9 @@ void ip_conntrack_cleanup(void)
 		schedule();
 		goto i_see_dead_people;
 	}
+	/* wait until all references to ip_conntrack_untracked are dropped */
+	while (atomic_read(&ip_conntrack_untracked.ct_general.use) > 1)
+		schedule();
 
 	kmem_cache_destroy(ip_conntrack_cachep);
 	kmem_cache_destroy(ip_conntrack_expect_cachep);