mlxsw: spectrum_router: Sanitize IPv6 FIB rules

We only allow FIB offload in the presence of default rules or an l3mdev
rule. In a similar fashion to IPv4 FIB rules, sanitize IPv6 rules.

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>