[SCSI] zfcp: remove union zfcp_req_data, use unit refcount for FCP commands

o union zfcp_req_data removed
o increment unit refcount when processing FCP commands
 (This fixes a theoretical race: When all scsi commands of a unit
  are aborted and the scsi_device is removed then the unit could be
  removed before all fsf_requests of that unit are completely processed.)

Signed-off-by: Andreas Herrmann <aherrman@de.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>