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/13045 Reviewed-by: Ian Lance Taylor <iant@golang.org>