udp: Remove partial csum code in TX.

udp: Remove partial csum code in TX.

UDP TX paths also have some code for UDP-Lite partial
checksum:

  * udplite_csum() in udp_send_skb() and udp_v6_send_skb()
  * udplite_getfrag() in udp_sendmsg() and udpv6_sendmsg()

Let's remove such code.

Now, we can use IPPROTO_UDP directly instead of sk->sk_protocol
or fl6->flowi6_proto for csum_tcpudp_magic() and csum_ipv6_magic().

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20260311052020.1213705-9-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>