pppoe: drop PPPOX_ZOMBIEs in pppoe_release

When PPPOE is running over a virtual ethernet interface (e.g., a
bonding interface) and the user tries to delete the interface in case
the PPPOE state is ZOMBIE, the kernel will loop forever while
unregistering net_device for the reference count is not decreased to
zero which should have been done with dev_put().

Signed-off-by: Xiaodong Xu <stid.smth@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>