xfrm: fix fragmentation for ipv4 xfrm tunnel

When generating the ip header for the transformed packet we just copy
the frag_off field of the ip header from the original packet to the ip
header of the new generated packet. If we receive a packet as a chain
of fragments, all but the last of the new generated packets have the
IP_MF flag set. We have to mask the frag_off field to only keep the
IP_DF flag from the original packet. This got lost with git commit
36cf9acf93e8561d9faec24849e57688a81eb9c5 ("[IPSEC]: Separate
inner/outer mode processing on output")

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/xfrm4_mode_tunnel.c

@@ -52,7 +52,7 @@ static int xfrm4_mode_tunnel_output(struct xfrm_state *x, struct sk_buff *skb)
 		IP_ECN_clear(top_iph);
 
 	top_iph->frag_off = (flags & XFRM_STATE_NOPMTUDISC) ?
-			    0 : XFRM_MODE_SKB_CB(skb)->frag_off;
+		0 : (XFRM_MODE_SKB_CB(skb)->frag_off & htons(IP_DF));
 	ip_select_ident(top_iph, dst->child, NULL);
 
 	top_iph->ttl = dst_metric(dst->child, RTAX_HOPLIMIT);