[VLAN]: Move some device intialization code to dev->init callback

Move some device initialization code to new dev->init callback to make
it shareable with netlink. Additionally this fixes a minor bug, dev->iflink
is set after registration, which causes an incorrect value in the initial
netlink message.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>