mlxsw: spectrum_router: Move IPv4 FIB info into a union in nexthop group struct

Instead of storing the FIB info as 'priv' when the nexthop group
represents an IPv4 nexthop group, simply store it as a FIB info with a
proper comment.

When nexthop objects are supported, this field will become a union with
the nexthop object's identifier.

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