net: netpoll: Initialize UDP checksum field before checksumming

commit f1fce08 ("netpoll: Eliminate redundant assignment") removed
the initialization of the UDP checksum, which was wrong and broke
netpoll IPv6 transmission due to bad checksumming.

udph->check needs to be set before calling csum_ipv6_magic().

Fixes: f1fce08 ("netpoll: Eliminate redundant assignment")
Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250620-netpoll_fix-v1-1-f9f0b82bc059@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: leitao

net/core/netpoll.c

@@ -432,6 +432,7 @@ int netpoll_send_udp(struct netpoll *np, const char *msg, int len)
 	udph->dest = htons(np->remote_port);
 	udph->len = htons(udp_len);
 
+	udph->check = 0;
 	if (np->ipv6) {
 		udph->check = csum_ipv6_magic(&np->local_ip.in6,
 					      &np->remote_ip.in6,
@@ -460,7 +461,6 @@ int netpoll_send_udp(struct netpoll *np, const char *msg, int len)
 		skb_reset_mac_header(skb);
 		skb->protocol = eth->h_proto = htons(ETH_P_IPV6);
 	} else {
-		udph->check = 0;
 		udph->check = csum_tcpudp_magic(np->local_ip.ip,
 						np->remote_ip.ip,
 						udp_len, IPPROTO_UDP,