skbuff: Add MSG_MORE flag to optimize tcp large packet transmission

When using sockmap for forwarding, the average latency for different packet sizes
after sending 10,000 packets is as follows:
size old(us) new(us)
512 56 55
1472 58 58
1600 106 81
3000 145 105
5000 182 125

Suggested-by: Eric Dumazet <[e-mail]>
Signed-off-by: Feng Yang <[e-mail]>
Reviewed-by: Eric Dumazet <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/core/skbuff.c

···
 static int __skb_send_sock(struct sock *sk, struct sk_buff *skb, int offset,
 			   int len, sendmsg_func sendmsg, int flags)
 {
+	int more_hint = sk_is_tcp(sk) ? MSG_MORE : 0;
 	unsigned int orig_len = len;
 	struct sk_buff *head = skb;
 	unsigned short fragidx;
···
 		kv.iov_len = slen;
 		memset(&msg, 0, sizeof(msg));
 		msg.msg_flags = MSG_DONTWAIT | flags;
+		if (slen < len)
+			msg.msg_flags |= more_hint;
 
 		iov_iter_kvec(&msg.msg_iter, ITER_SOURCE, &kv, 1, slen);
 		ret = INDIRECT_CALL_2(sendmsg, sendmsg_locked,
···
 					     flags,
 			};
 
+			if (slen < len)
+				msg.msg_flags |= more_hint;
 			bvec_set_page(&bvec, skb_frag_page(frag), slen,
 				      skb_frag_off(frag) + offset);
 			iov_iter_bvec(&msg.msg_iter, ITER_SOURCE, &bvec, 1,