mlxsw: spectrum_router: Split nexthop group configuration to a different struct

Currently, the individual nexthops member in the group and attributes of
the group (e.g., its type) are stored in the same struct (i.e., 'struct
mlxsw_sp_nexthop_group'). This is fine since the individual nexthops
cannot change during the lifetime of the group.

With nexthop objects this is no longer the case. An existing nexthop
group can be replaced to use a new set of nexthops. Creating a new
struct whenever a group is replaced entails replacing the group pointer
of all the routes (i.e., 'struct mlxsw_sp_fib_entry') using the group.

Avoid this inefficient step by splitting the nexthop group configuration
to a different struct (i.e., 'struct mlxsw_sp_nexthop_group_info').
When a nexthop group is replaced a new group info struct is created and
the individual rotues do not need to be touched.

Illustration after the change:

  mlxsw_sp_fib_entry    mlxsw_sp_nexthop_group    mlxsw_sp_nexthop_group_info
+-------------------+  +----------------------+  +---------------------------+
| nh_group;         +--> nhgi;                +-->                           |
|                   |  |                      |  |                           |
+-------------------+  +----------------------+  +---------------------------+

No functional changes intended.
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>