scsi: be2iscsi: Add checks to validate completions

Added check in beiscsi_process_cq for pio_handle.
pio_handle is cleared in beiscsi_put_wrb_handle.
This catches any case where task gets cleaned up just before completion.

Use back_lock before accessing pio_handle.

Signed-off-by: Jitendra Bhivare <jitendra.bhivare@broadcom.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>