netpoll: factor out UDP header setup into push_udp() helper

Move UDP header construction from netpoll_send_udp() into a new
static helper function push_udp(). This completes the protocol
layer refactoring by:

1. Creating a dedicated helper for UDP header assembly
2. Removing UDP-specific logic from the main send function
3. Establishing a consistent pattern with existing IPv4/IPv6 helpers:
   - push_udp()
   - push_ipv4()
   - push_ipv6()

The change improves code organization and maintains the encapsulation
pattern established in previous refactorings.

Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: NipaLocal <nipa@local>

Author: leitao

net/core/netpoll.c

@@ -473,11 +473,28 @@ static void push_ipv4(struct netpoll *np, struct sk_buff *skb, int len)
 	eth->h_proto = htons(ETH_P_IP);
 }
 
+static void push_udp(struct netpoll *np, struct sk_buff *skb, int len)
+{
+	struct udphdr *udph;
+	int udp_len;
+
+	udp_len = len + sizeof(struct udphdr);
+
+	skb_push(skb, sizeof(struct udphdr));
+	skb_reset_transport_header(skb);
+
+	udph = udp_hdr(skb);
+	udph->source = htons(np->local_port);
+	udph->dest = htons(np->remote_port);
+	udph->len = htons(udp_len);
+
+	netpoll_udp_checksum(np, skb, len);
+}
+
 int netpoll_send_udp(struct netpoll *np, const char *msg, int len)
 {
 	int total_len, ip_len, udp_len;
 	struct sk_buff *skb;
-	struct udphdr *udph;
 	struct ethhdr *eth;
 
 	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
@@ -499,14 +516,7 @@ int netpoll_send_udp(struct netpoll *np, const char *msg, int len)
 	skb_copy_to_linear_data(skb, msg, len);
 	skb_put(skb, len);
 
-	skb_push(skb, sizeof(struct udphdr));
-	skb_reset_transport_header(skb);
-	udph = udp_hdr(skb);
-	udph->source = htons(np->local_port);
-	udph->dest = htons(np->remote_port);
-	udph->len = htons(udp_len);
-
-	netpoll_udp_checksum(np, skb, len);
+	push_udp(np, skb, len);
 	if (np->ipv6)
 		push_ipv6(np, skb, len);
 	else