netdevsim: Add devlink resource for nexthops

netdevsim: Add devlink resource for nexthops

The Spectrum ASIC has a dedicated table where nexthops (i.e., adjacency
entries) are populated. The size of this table can be controlled via
devlink-resource.

Add such a resource to netdevsim so that its occupancy will reflect the
number of nexthop objects currently programmed to the device.

By limiting the size of the resource, error paths could be exercised and
tested.

Example output:

# devlink resource show netdevsim/netdevsim10
netdevsim/netdevsim10:
  name IPv4 size unlimited unit entry size_min 0 size_max unlimited size_gran 1 dpipe_tables none
    resources:
      name fib size unlimited occ 4 unit entry size_min 0 size_max unlimited size_gran 1 dpipe_tables none
      name fib-rules size unlimited occ 3 unit entry size_min 0 size_max unlimited size_gran 1 dpipe_tables none
  name IPv6 size unlimited unit entry size_min 0 size_max unlimited size_gran 1 dpipe_tables none
    resources:
      name fib size unlimited occ 1 unit entry size_min 0 size_max unlimited size_gran 1 dpipe_tables none
      name fib-rules size unlimited occ 2 unit entry size_min 0 size_max unlimited size_gran 1 dpipe_tables none
  name nexthops size unlimited occ 0 unit entry size_min 0 size_max unlimited size_gran 1 dpipe_tables none

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>