udp: avoid csum_partial() for validated skb

In commit e6afc8ac ("udp: remove headers from UDP packets before
queueing"), udp_csum_pull_header() helper was added but missed fact
that CHECKSUM_UNNECESSARY packets were now converted to CHECKSUM_NONE
and skb->csum_valid was set to 1 for them.

Since csum_partial() is quite expensive, even for 8-byte area, it is
worth adding a test.

We also can use skb->data instead of udp_hdr() as we are pulling
UDP headers, as it is sightly faster.
Signed-off-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/net/udp.h

@@ -160,8 +160,8 @@ void udp_set_csum(bool nocheck, struct sk_buff *skb,
 
 static inline void udp_csum_pull_header(struct sk_buff *skb)
 {
-	if (skb->ip_summed == CHECKSUM_NONE)
-		skb->csum = csum_partial(udp_hdr(skb), sizeof(struct udphdr),
+	if (!skb->csum_valid && skb->ip_summed == CHECKSUM_NONE)
+		skb->csum = csum_partial(skb->data, sizeof(struct udphdr),
 					 skb->csum);
 	skb_pull_rcsum(skb, sizeof(struct udphdr));
 	UDP_SKB_CB(skb)->cscov -= sizeof(struct udphdr);