net: devlink: convert port_list into xarray

Some devlink instances may contain thousands of ports. Storing them in
linked list and looking them up is not scalable. Convert the linked list
into xarray.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>