net/smc: remember PNETID of IB device for later device matching

The PNETID is needed to find an alternate link for a link group.
Save the PNETID of the link that is used to create the link group for
later device matching.

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>