tcp: Move skb_steal_sock() to request_sock.h

tcp: Move skb_steal_sock() to request_sock.h

We will support arbitrary SYN Cookie with BPF.

If BPF prog validates ACK and kfunc allocates a reqsk, it will
be carried to TCP stack as skb->sk with req->syncookie 1.

In skb_steal_sock(), we need to check inet_reqsk(sk)->syncookie
to see if the reqsk is created by kfunc.  However, inet_reqsk()
is not available in sock.h.

Let's move skb_steal_sock() to request_sock.h.

While at it, we refactor skb_steal_sock() so it returns early if
skb->sk is NULL to minimise the following patch.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20240115205514.68364-3-kuniyu@amazon.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>