net/smc: introduce System Enterprise ID (SEID)

SMCD version 2 defines a System Enterprise ID (short SEID).
This patch contains the SEID creation and adds the callback to
retrieve the created SEID.

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>