IPv6: fix to set device name when new IPv6 over IPv6 tunnel device is created.

When the user creates IPv6 over IPv6 tunnel, the device name created
by the kernel isn't set to t->parm.name, which is referred as the
result of ioctl().

Signed-off-by: Noriaki TAKAMIYA <takamiya@po.ntts.co.jp>
Signed-off-by: David S. Miller <davem@davemloft.net>