tipc: orphan sock in tipc_release()

Before we unlock the sock in tipc_release(), we have to
detach sk->sk_socket from sk, otherwise a parallel
tipc_sk_fill_sock_diag() could stil read it after we
free this socket.

Fixes: c30b70de ("tipc: implement socket diagnostics for AF_TIPC")
Reported-and-tested-by:  <syzbot+48804b87c16588ad491d@syzkaller.appspotmail.com>
Cc: Jon Maloy <jon.maloy@ericsson.com>
Cc: Ying Xue <ying.xue@windriver.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Acked-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>