ipv6: call ipv6_proxy_select_ident instead of ipv6_select_ident in udp6_ufo_fragment

Matt Grant reported frequent crashes in ipv6_select_ident when
udp6_ufo_fragment is called from openvswitch on a skb that doesn't
have a dst_entry set.

ipv6_proxy_select_ident generates the frag_id without using the dst
associated with the skb.  This approach was suggested by Vladislav
Yasevich.

Fixes: 0508c07f ("ipv6: Select fragment id during UFO segmentation if not set.")
Cc: Vladislav Yasevich <vyasevic@redhat.com>
Reported-by: Matt Grant <matt@mattgrant.net.nz>
Tested-by: Matt Grant <matt@mattgrant.net.nz>
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Acked-by: Vladislav Yasevich <vyasevic@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/udp_offload.c

@@ -112,11 +112,9 @@ static struct sk_buff *udp6_ufo_fragment(struct sk_buff *skb,
 		fptr = (struct frag_hdr *)(skb_network_header(skb) + unfrag_ip6hlen);
 		fptr->nexthdr = nexthdr;
 		fptr->reserved = 0;
-		if (skb_shinfo(skb)->ip6_frag_id)
-			fptr->identification = skb_shinfo(skb)->ip6_frag_id;
-		else
-			ipv6_select_ident(fptr,
-					  (struct rt6_info *)skb_dst(skb));
+		if (!skb_shinfo(skb)->ip6_frag_id)
+			ipv6_proxy_select_ident(skb);
+		fptr->identification = skb_shinfo(skb)->ip6_frag_id;
 
 		/* Fragment the skb. ipv6 header and the remaining fields of the
 		 * fragment header are updated in ipv6_gso_segment()