udp: Refactor udp_read_skb()

Delete the unnecessary while loop in udp_read_skb() for readability.
Additionally, since recv_actor() cannot return a value greater than
skb->len (see sk_psock_verdict_recv()), remove the redundant check.
Suggested-by: Cong Wang <cong.wang@bytedance.com>
Signed-off-by: Peilin Ye <peilin.ye@bytedance.com>
Link: https://lore.kernel.org/r/343b5d8090a3eb764068e9f1d392939e2b423747.1663909008.git.peilin.ye@bytedance.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/udp.c

@@ -1801,41 +1801,29 @@ EXPORT_SYMBOL(__skb_recv_udp);
 
 int udp_read_skb(struct sock *sk, skb_read_actor_t recv_actor)
 {
-	int copied = 0;
-
-	while (1) {
-		struct sk_buff *skb;
-		int err, used;
-
-		skb = skb_recv_udp(sk, MSG_DONTWAIT, &err);
-		if (!skb)
-			return err;
+	struct sk_buff *skb;
+	int err, copied;
 
-		if (udp_lib_checksum_complete(skb)) {
-			__UDP_INC_STATS(sock_net(sk), UDP_MIB_CSUMERRORS,
-					IS_UDPLITE(sk));
-			__UDP_INC_STATS(sock_net(sk), UDP_MIB_INERRORS,
-					IS_UDPLITE(sk));
-			atomic_inc(&sk->sk_drops);
-			kfree_skb(skb);
-			continue;
-		}
+try_again:
+	skb = skb_recv_udp(sk, MSG_DONTWAIT, &err);
+	if (!skb)
+		return err;
 
-		WARN_ON_ONCE(!skb_set_owner_sk_safe(skb, sk));
-		used = recv_actor(sk, skb);
-		if (used <= 0) {
-			if (!copied)
-				copied = used;
-			kfree_skb(skb);
-			break;
-		} else if (used <= skb->len) {
-			copied += used;
-		}
+	if (udp_lib_checksum_complete(skb)) {
+		int is_udplite = IS_UDPLITE(sk);
+		struct net *net = sock_net(sk);
 
+		__UDP_INC_STATS(net, UDP_MIB_CSUMERRORS, is_udplite);
+		__UDP_INC_STATS(net, UDP_MIB_INERRORS, is_udplite);
+		atomic_inc(&sk->sk_drops);
 		kfree_skb(skb);
-		break;
+		goto try_again;
 	}
 
+	WARN_ON_ONCE(!skb_set_owner_sk_safe(skb, sk));
+	copied = recv_actor(sk, skb);
+	kfree_skb(skb);
+
 	return copied;
 }
 EXPORT_SYMBOL(udp_read_skb);