net/smc: create improved SMC-R link_uid

The link_uid of an SMC-R link is exchanged between SMC peers and its
value can be used for debugging purposes. Create a unique link_uid
during link initialization and use it in communication with SMC-R peers.

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