net: core: add __netdev_upper_dev_unlink()

The netdev_upper_dev_unlink() has to work differently according to flags.
This idea is the same with __netdev_upper_dev_link().

In the following patches, new flags will be added.

Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>