tcp: Restrict SO_TXREHASH to TCP socket.

sk->sk_txrehash is only used for TCP.

Let's restrict SO_TXREHASH to TCP to reflect this.

Later, we will make sk_txrehash a part of the union for other
protocol families.

Note that we need to modify BPF selftest not to get/set
SO_TEREHASH for non-TCP sockets.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>