scsi: virtio_scsi: Use complete() instead complete_all()

There is only one waiter for the completion, therefore there is no need
to use complete_all(). Let's make that clear by using complete() instead
of complete_all().

The usage pattern of the completion is:

waiter context                          waker context

virtscsi_tmf()
  DECLARE_COMPLETION_ONSTACK()
  virtscsi_kick_cmd()
  wait_for_completion()

                                        virtscsi_complete_free()
                                          complete()

Signed-off-by: Daniel Wagner <daniel.wagner@bmw-carit.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>