mlxsw: Propagate extack down to register_fib_notifier()

During the devlink reaload the extack is present, so propagate it all
the way down to register_fib_notifier() call in spectrum_router.c.

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