Use unsafe.Offsetof() instead of magic number. (#3)

* Use Offsetof() instead of magic number.
* Remove useless comments.

common/header_test.go

+package common
+
+import (
+	"fmt"
+	"testing"
+	"unsafe"
+)
+
+func TestHeaderPrint(t *testing.T) { 
+	header := Header{} 
+
+	infoMsg := ` 
+		header layout info: size: %d, align: %d
+		field          offset         size
+		--------------+--------------+--------
+		Tag            %03d           %d
+		Version        %03d           %d
+		DiskCaps       %03d           %d
+		DataBlockSize  %03d           %d
+		RangeCaps      %03d           %d
+		RangeCoverage  %03d           %d
+		RangeOffset    %03d           %d
+		HashOffset     %03d           %d
+		DataOffset     %03d           %d
+		DataCount      %03d           %d
+		AllocOffset    %03d           %d
+		ResolveOffset  %03d           %d
+		Reserved       %03d           %d
+		` 
+
+	fmt.Printf(infoMsg,
+		unsafe.Sizeof(header),					unsafe.Alignof(header), 
+		unsafe.Offsetof(header.Tag),			unsafe.Sizeof(header.Tag),
+		unsafe.Offsetof(header.Version),		unsafe.Sizeof(header.Version),
+		unsafe.Offsetof(header.DiskCaps),		unsafe.Sizeof(header.DiskCaps),
+		unsafe.Offsetof(header.DataBlockSize),	unsafe.Sizeof(header.DataBlockSize),
+		unsafe.Offsetof(header.RangeCaps),		unsafe.Sizeof(header.RangeCaps),
+		unsafe.Offsetof(header.RangeCoverage),	unsafe.Sizeof(header.RangeCoverage),
+		unsafe.Offsetof(header.RangeOffset),	unsafe.Sizeof(header.RangeOffset),
+		unsafe.Offsetof(header.HashOffset),		unsafe.Sizeof(header.HashOffset),
+		unsafe.Offsetof(header.DataOffset),		unsafe.Sizeof(header.DataOffset),
+		unsafe.Offsetof(header.DataCount),		unsafe.Sizeof(header.DataCount),
+		unsafe.Offsetof(header.AllocOffset),	unsafe.Sizeof(header.AllocOffset),
+		unsafe.Offsetof(header.ResolveOffset),	unsafe.Sizeof(header.ResolveOffset),
+		unsafe.Offsetof(header.Reserved),		unsafe.Sizeof(header.Reserved))
+}
\ No newline at end of file

yottadisk.go

@@ -100,8 +100,8 @@ func (disk *YottaDisk) writeData(idx uint32, key ydcommon.IndexTableKey, dataOff
 			return err
 		}
 
-		// Update data count. WARNING: use magic number
-		writer.Seek(48, io.SeekStart)
+		// Update data count.
+		writer.Seek((int64)(unsafe.Offsetof(disk.meta.DataCount)), io.SeekStart)
 		_, err = writer.Write(dcBuf.Bytes())
 		if err != nil {
 			return err