af_iucv: exploit target message class support of IUCV

The first 4 bytes of data to be sent are stored additionally into
the message class field of the send request. A receiving target
program (not an af_iucv socket program) can make use of this
information to pre-screen incoming messages.

Signed-off-by: Ursula Braun <braunu@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>