mlxsw: spectrum_dpipe: Add adjacency group size

The adjacency group size is part of the match on the adjacency group and
should therefore be exposed using dpipe.

When non-equal-cost multi-path support will be introduced, the group's
size will help users understand the exact number of adjacency entries
each nexthop occupies, as a nexthop will no longer correspond to a
single entry.

The output for a multi-path route with two nexthops, one with weight 255
and the second 1 will be:

Example:

$ devlink dpipe table dump pci/0000:01:00.0 name mlxsw_adj
pci/0000:01:00.0:
  index 0
  match_value:
    type field_exact header mlxsw_meta field adj_index value 65536
    type field_exact header mlxsw_meta field adj_size value 512
    type field_exact header mlxsw_meta field adj_hash_index value 0
  action_value:
    type field_modify header ethernet field destination mac value e4:1d:2d:a5:f3:64
    type field_modify header mlxsw_meta field erif_port mapping ifindex mapping_value 3 value 1

  index 1
  match_value:
    type field_exact header mlxsw_meta field adj_index value 65536
    type field_exact header mlxsw_meta field adj_size value 512
    type field_exact header mlxsw_meta field adj_hash_index value 510
  action_value:
    type field_modify header ethernet field destination mac value e4:1d:2d:a5:f3:65
    type field_modify header mlxsw_meta field erif_port mapping ifindex mapping_value 4 value 2

Thus, the first nexthop occupies 510 adjacency entries and the second 2,
which leads to a ratio of 255 to 1.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>