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>