ixgbe: add a refcnt when turning on/off FCoE offload capability

The FCoE offload is enabled/disabled per adapter, but upper FCoE protocol
stack could have multiple FCoE instances created on the same physical network
interface, e.g., FCoE on multiple VLAN interfaces on the same physical
network interface. In this case we want to turn on FCoE offload at the first
request from ndo_fcoe_enable() but only turn off FCoE offload at the very last
call to ndo_fcoe_disable(). This is fixed by adding a refcnt in the per adapter
FCoE structure and tear down FCoE offload when refcnt decrements to zero.

Signed-off-by: Yi Zou <yi.zou@intel.com>
Tested-by: Ross Brattain <ross.b.brattain@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>