af_iucv: allow retrieval of maximum message size

For HS transport the maximum message size depends on the MTU-size
of the HS-device bound to the AF_IUCV socket. This patch adds a
getsockopt option MSGSIZE returning the maximum message size that
can be handled for this AF_IUCV socket.

Signed-off-by: Ursula Braun <ursula.braun@de.ibm.com>
Signed-off-by: Frank Blaschka <frank.blaschka@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>