Files of type Symlink are not written correctly

[antoineco]

The library is able to deserialize Header structs from Symlink files:

cpio/svr4.go

Lines 90 to 101 in b4d3577

	// read link name
	if hdr.Mode&^ModePerm == ModeSymlink {
	if hdr.Size < 1 || hdr.Size > svr4MaxNameSize {
	return nil, ErrHeader
	}
	b := make([]byte, hdr.Size)
	if _, err := io.ReadFull(r, b); err != nil {
	return nil, err
	}
	hdr.Linkname = string(b)
	hdr.Size = 0
	}

However, it completely ignores the Linkname attribute while writing the header, resulting in zero-sized files without target.

Note that using writer.Write(target) as a workaround isn't an option because the method isn't supported for files other than Regular:

cpio/writer.go

Lines 103 to 105 in b4d3577

	// Calling Write on special types like TypeLink, TypeSymlink, TypeChar,
	// TypeBlock, TypeDir, and TypeFifo returns (0, ErrWriteTooLong) regardless of
	// what the Header.Size claims.