fcoe: implement FIP VLAN responder

When running in VN2VN mode there is no central instance which would send
out any FIP VLAN discovery notifications. So this patch adds a new sysfs
attribute 'fip_vlan_responder' which will activate a FIP VLAN discovery
responder.

Signed-off-by: Hannes Reinecke <hare@suse.com>
Acked-by: Johannes Thumshirn <jth@kernel.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>