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: 32dce968dd987a (ipv6: Allow for partial checksums on non-ufo packets)
Reported-by: Sabrina Dubroca <[e-mail]>
Tested-by: Sabrina Dubroca <[e-mail]>
Signed-off-by: Vladislav Yasevich <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/ipv6/ip6_output.c

···
 	/* 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)