mlxsw: spectrum_router: Flush FIB tables during fini

Since commit b45f64d1 ("mlxsw: spectrum_router: Use FIB notifications
instead of switchdev calls") we reflect to the device the entire FIB
table and not only FIBs that point to netdevs created by the driver.

During module removal, FIBs of the second type are removed following
NETDEV_UNREGISTER events sent. The other FIBs are still present in both
the driver's cache and the device's table.

Fix this by iterating over all the FIB tables in the device and flush
them. There's no need to take locks, as we're the only writer.

Fixes: b45f64d1 ("mlxsw: spectrum_router: Use FIB notifications instead of switchdev calls")
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>