ipvlan: assign unique dev-id for each slave device.

IPvlan setup uses one mac-address (of master). The IPv6 link-local
addresses are derived using the mac-address on the link. Lack of
dev-ids makes these link-local addresses same for all slaves including
that of master device. dev-ids are necessary to add differentiation
when L2 address is shared.

Signed-off-by: Mahesh Bandewar <maheshb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>