driver: macvlan: Remove the rcu member of macvlan_port

When free macvlan_port in macvlan_port_destroy, it is safe to free
directly because netdev_rx_handler_unregister could enforce one
grace period.
So it is unnecessary to use kfree_rcu for macvlan_port.

Signed-off-by: Gao Feng <fgao@ikuai8.com>
Signed-off-by: David S. Miller <davem@davemloft.net>