[SCSI] libfcoe: Remove stale fcoe-netdev entries

When L2 driver is unloaded, libfcoe_destroy tries to access the fcoe
transport structure matching the netdev. However, since the netdev is
unregistered by that time, it fails to do so. Hence the stale mappings
exists in the fcoe-netdev list. Handle NETDEV_UREGISTER device
notification mechanism to remove the stale fcoe-netdev mapping.

Signed-off-by: Bhanu Prakash Gollapudi <bprakash@broadcom.com>
Signed-off-by: Robert Love <robert.w.love@intel.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>