vlan: link the upper neighbour only after registering

Otherwise users might access it without being fully registered, as per
sysfs - it only inits in register_netdevice(), so is unusable till it is
called.

CC: Patrick McHardy <kaber@trash.net>
CC: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Veaceslav Falico <vfalico@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>