scsi: tcmu: fix use after free

Fixes: a94a2572 ("scsi: tcmu: avoid cmd/qfull timers updated whenever a new cmd comes")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Mike Christie <mchristi@redhat.com>
Signed-off-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>