mlxsw: spectrum_router: Don't assume neighbour type

Thankfully, the neighbour subsystem is agnostic to the upper protocol
and used by both IPv4 and IPv6. By removing assumptions regarding the
neighbour type we can thus re-use much of the neighbour-related code for
both IPv4 and IPv6.

For each nexthop, store its gateway IP and for nexthop group store the
neighbour table used by its nexthops.

Use this information throughout the code and remove assumption about the
neighbour type.

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>