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 <edumazet@google.com>
Signed-off-by: Feng Yang <yangfeng@kylinos.cn>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250708054053.39551-1-yangfeng59949@163.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>