net/af_iucv: don't lookup the socket on TX notification

Whoever called iucv_sk(sk)->sk_txnotify() must already know that they're
dealing with an af_iucv socket.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>