ip/ipvlan: enhance ability to add mode flags to existing modes

ip/ipvlan: enhance ability to add mode flags to existing modes

IPvlan supported bridge-only functionality prior to commits
a190d04db937 ('ipvlan: introduce 'private' attribute for all
existing modes.') and fe89aa6b250c ('ipvlan: implement VEPA mode').
These two commits allow to configure the VEPA and private modes now.
This patch adds those options in ip command.

e.g.
  bash:~# ip link add link eth0 name ipvl0 type ipvlan mode l2 private
  -or-
  bash:~# ip link add link eth0 type ipvl0 type ipvlan mode l2 vepa

Also the output will reflect the mode and the mode-flag accordingly.
e.g.
  bash:~# ip -details link show ipvl0
  4: ipvl0@eth0: <BROADCAST,MULTICAST> mtu 1500 qdisc ...
     link/ether 00:1a:11:44:a5:3e brd ff:ff:ff:ff:ff:ff promiscuity 0
     ipvlan  mode l2 private addrgenmode eui64 numtxqueues 1 ...

Signed-off-by: Mahesh Bandewar <maheshb@google.com>