net: add missing descriptions in net_device_priv_flags

The flags IFF_XMIT_DST_RELEASE_PERM, IFF_IPVLAN_MASTER and
IFF_IPVLAN_SLAVE are missing descriptions for the Documentation. Adding
them.
Signed-off-by: Luis de Bethencourt <luisbg@osg.samsung.com>
Suggested-by: Benjamin Poirier <benjamin.poirier@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -1320,6 +1320,10 @@ struct net_device_ops {
  * @IFF_LIVE_ADDR_CHANGE: device supports hardware address
  *	change when it's running
  * @IFF_MACVLAN: Macvlan device
+ * @IFF_XMIT_DST_RELEASE_PERM: IFF_XMIT_DST_RELEASE not taking into account
+ *	underlying stacked devices
+ * @IFF_IPVLAN_MASTER: IPvlan master device
+ * @IFF_IPVLAN_SLAVE: IPvlan slave device
  * @IFF_L3MDEV_MASTER: device is an L3 master device
  * @IFF_NO_QUEUE: device can run without qdisc attached
  * @IFF_OPENVSWITCH: device is a Open vSwitch master