net/smc: take link down instead of terminating the link group

Use the introduced link down processing in all places where the link
group is terminated and take down the affected link only.

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>