mlxsw: spectrum_router: Pass multiple routes to work item

Prepare the driver to process IPv6 multipath notifications by passing an
array of 'struct fib6_info' instead of just one route.

A reference is taken on each sibling route in order to prevent them from
being freed until they are processed by the workqueue.

v2:
* Remove 'multipath_rt' usage

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