net/smc: split CLC confirm/accept data to be sent

When sending CLC confirm and CLC accept, separate the trailing
part of the message from the initial part (to be prepared for
future first contact extension).

Signed-off-by: Ursula Braun <ubraun@linux.ibm.com>
Signed-off-by: Karsten Graul <kgraul@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>