scsi: fix cancellation when I/O was completed but DMA was not.

scsi: fix cancellation when I/O was completed but DMA was not.

Commit d577646 (scsi: Introduce scsi_req_cancel_complete, 2014-09-25)
was supposed to have no semantic change, but it missed a case.  When
r->aiocb has already been NULLed, but DMA was not complete and the
SCSI layer was waiting for scsi_req_continue, after the patch the
SCSI layer will not call the .cancel callback of SCSIBusInfo.

Fixes: d5776465ee9a55815792efa34d79de240f4ffd99
Cc: [email protected]
Reported-by: Dr. David Alan Gilbert <[email protected]>
Tested-by: Dr. David Alan Gilbert <[email protected]>
Reviewed-by: Fam Zheng <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>