net/smc: add listen processing for SMC-Rv2

Implement the server side of the SMC-Rv2 processing. Process incoming
CLC messages, find eligible devices and check for a valid route to the
remote peer.

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