iucv: prevent information leak in iucv_message

Initialize storage for the future IUCV header that will be included
in the transmitted packet. Some of the header fields are unused with
HiperSockets transport, and will contain data left from some other
functions.

Signed-off-by: Eugene Crosser <Eugene.Crosser@ru.ibm.com>
Signed-off-by: Ursula Braun <ubraun@linux.vnet.ibm.com>
Reviewed-by: Thomas Richter <tmricht@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>