tipc: use the right skb in tipc_sk_fill_sock_diag()

Commit 4b2e6877

 ("tipc: Fix namespace violation in tipc_sk_fill_sock_diag")
tried to fix the crash but failed, the crash is still 100% reproducible
with it.

In tipc_sk_fill_sock_diag(), skb is the diag dump we are filling, it is not
correct to retrieve its NETLINK_CB(), instead, like other protocol diag,
we should use NETLINK_CB(cb->skb).sk here.

Reported-by:  <syzbot+326e587eff1074657718@syzkaller.appspotmail.com>
Fixes: 4b2e6877 ("tipc: Fix namespace violation in tipc_sk_fill_sock_diag")
Fixes: c30b70de

 (tipc: implement socket diagnostics for AF_TIPC)
Cc: GhantaKrishnamurthy MohanKrishna <mohan.krishna.ghanta.krishnamurthy@ericsson.com>
Cc: Jon Maloy <jon.maloy@ericsson.com>
Cc: Ying Xue <ying.xue@windriver.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>