Added support to ipoib rtnl ops through which one can create, configure,
query and delete IPoIB devices, for example

 $ ip link add link ib0.8001 name ib0.8001 type ipoib pkey 0x8001
 $ ip link add link ib0.1 name ib0.1 type ipoib mode connected
 $ ip --details link show dev ib0.1

Cc: Patrick McHardy <kaber@trash.net>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>