mlxsw: spectrum_router: Provide offload indication using nexthop flags

In a similar fashion to previous patch, use the nexthop flags to provide
offload indication instead of the FIB info's flags.

In case a nexthop in a multipath route can't be offloaded (gateway's MAC
can't be resolved, for example), then its offload flag isn't set.

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