fs: use io.Copy because go supports CopyFileRange

[fuweid]

REF: golang/go@7be3f09

[dmcgowan]

I wonder if we could just get rid of this function all together, for unix we still do

func copyFileContent(dst, src *os.File) error {
	buf := bufferPool.Get().(*[]byte)
	_, err := io.CopyBuffer(dst, src, *buf)
	bufferPool.Put(buf)

	return err
}

Based on the Go code though, this buffer will never be used for os.File to os.File copying though
https://github.com/golang/go/blob/master/src/os/file.go#L161

[fuweid]

I wonder if we could just get rid of this function all together

If the go runtime move the ReadFrom into linux platform, I guess it will be possible.

Updated. Please take a look.

[dmcgowan]

The buffer didn't even end up getting used before, should we even bother with it rather than checking OS. It could be possible for Go to implement copy optimizations on other platforms as well.

[fuweid]

Updated. Let's use io.Copy directly.