mlxsw: spectrum_router: Demultiplex FIB event based on family

The FIB notification block currently only handles IPv4 events, but we
want to start handling IPv6 events soon, so lay the groundwork now.

Do that by preparing the work item and process it according to the
notified address family.

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>