net: add skb_set_owner_edemux() helper

This can be used to attach a socket to an skb,
taking a reference on sk->sk_refcnt.

This helper might be a NOP if sk->sk_refcnt is zero.

Use it from tcp_make_synack().

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Brian Vazquez <brianvv@google.com>
Link: https://patch.msgid.link/20241010174817.1543642-4-edumazet@google.com


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