mlxsw: spectrum_router: Store FIB info in route

When needed, IPv4 routes fetch the FIB info (i.e., 'struct fib_info')
from their associated nexthop group. This will not work when the nexthop
group represents a nexthop object (i.e., 'struct nexthop'), as it will
only have access to the nexthop's identifier.

Instead, store the FIB info in the route itself.

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