[VLAN]: Fix net_device leak.

In "[VLAN]: Move device registation to seperate function" (commit
e89fe42c), a pile of code got moved
to register_vlan_dev(), including grabbing a reference to underlying
device.  However, original dev_hold() had been left behind, so we
leak a reference to net_device now...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>