ipv6: Partial checksum only UDP packets

ip6_append_data is used by other protocols and some of them can't
be partially checksummed.  Only partially checksum UDP protocol.

Fixes: 32dce968 (ipv6: Allow for partial checksums on non-ufo packets)
Reported-by: NSabrina Dubroca <sd@queasysnail.net>
Tested-by: NSabrina Dubroca <sd@queasysnail.net>
Signed-off-by: NVladislav Yasevich <vyasevic@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/ip6_output.c

@@ -1273,7 +1273,7 @@ static int __ip6_append_data(struct sock *sk,
 	/* If this is the first and only packet and device
 	 * supports checksum offloading, let's use it.
 	 */
-	if (!skb &&
+	if (!skb && sk->sk_protocol == IPPROTO_UDP &&
 	    length + fragheaderlen < mtu &&
 	    rt->dst.dev->features & NETIF_F_V6_CSUM &&
 	    !exthdrlen)