sctp: hold transport instead of assoc in sctp_diag

In sctp_transport_lookup_process(), Commit 1cceda784980 ("sctp: fix
the issue sctp_diag uses lock_sock in rcu_read_lock") moved cb() out
of rcu lock, but it put transport and hold assoc instead, and ignore
that cb() still uses transport. It may cause a use-after-free issue.

This patch is to hold transport instead of assoc there.

Fixes: 1cceda784980 ("sctp: fix the issue sctp_diag uses lock_sock in rcu_read_lock")
Signed-off-by: Xin Long <[e-mail]>
Acked-by: Marcelo Ricardo Leitner <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/sctp/socket.c

···
 	if (!transport || !sctp_transport_hold(transport))
 		goto out;
 
-	sctp_association_hold(transport->asoc);
-	sctp_transport_put(transport);
-
 	rcu_read_unlock();
 	err = cb(transport, p);
-	sctp_association_put(transport->asoc);
+	sctp_transport_put(transport);
 
 out:
 	return err;