net/smc: introduce link_idx for link group array

The link_id is the index of the link in the array of the link group.
When a link in the array is reused for a new link, a different unique
link_id should be used, otherwise the index in the array could collide
with the previous link at this array position.
Use a new variable link_idx as array index, and make link_id an
increasing unique id value.

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>