archive/tar: don't treat multiple file system links as a tar hardlink

Do not assume that if stat shows multiple links that we should mark the
file as a hardlink in the tar format.  If the hardlink link was not
referenced, this caused a link to "/".  On an overlay file system, all
files have multiple links.

The caller must keep the inode references and set TypeLink, Size = 0,
and LinkName themselves.

Change-Id: I873b8a235bc8f8fbb271db74ee54232da36ca013
Reviewed-on: https://go-review.googlesource.com/13045Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/archive/tar/stat_unix.go

@@ -28,10 +28,5 @@ func statUnix(fi os.FileInfo, h *Header) error {
 	h.AccessTime = statAtime(sys)
 	h.ChangeTime = statCtime(sys)
 	// TODO(bradfitz): major/minor device numbers?
-	if fi.Mode().IsRegular() && sys.Nlink > 1 {
-		h.Typeflag = TypeLink
-		h.Size = 0
-		// TODO(vbatts): Linkname?
-	}
 	return nil
 }