mlxsw: spectrum_router: Add support for IPv6 non-equal-cost multipath

Since commit eb789980

 ("mlxsw: spectrum_router: Populate adjacency
entries according to weights") the driver includes support for
non-equal-cost multipath, but IPv4 nexthops were the only user.

Now that the kernel supports weighted IPv6 nexthops, we can extend the
driver to support it as well.

This is done by assigning each nexthop its configured weight, so that it
will be populated accordingly in the device's adjacency table. The
`weight` parameter is also taken into account when comparing nexthop
groups in order not to consolidate non-identical groups.

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>