ipv6: only apply anti-spoofing checks to not-pointopoint tunnels

Because of commit 218774dc ("ipv6: add
anti-spoofing checks for 6to4 and 6rd") the sit driver dropped packets
for 2002::/16 destinations and sources even when configured to work as a
tunnel with fixed endpoint. We may only apply the 6rd/6to4 anti-spoofing
checks if the device is not in pointopoint mode.

This was an oversight from me in the above commit, sorry.  Thanks to
Roman Mamedov for reporting this!
Reported-by: NRoman Mamedov <rm@romanrm.ru>
Cc: David Miller <davem@davemloft.net>
Cc: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: NHannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/sit.c

@@ -593,7 +593,7 @@ static int ipip6_rcv(struct sk_buff *skb)
 				tunnel->dev->stats.rx_errors++;
 				goto out;
 			}
-		} else {
+		} else if (!(tunnel->dev->flags&IFF_POINTOPOINT)) {
 			if (is_spoofed_6rd(tunnel, iph->saddr,
 					   &ipv6_hdr(skb)->saddr) ||
 			    is_spoofed_6rd(tunnel, iph->daddr,