Fix SF bug # 1330039, patch # 1331635 from Lars Gustaebel (tarfile maintainer)

Problem: if two files are assigned the same inode
number by the filesystem, the second one will be added
as a hardlink to the first, which means that the
content will be lost.

The patched code checks if the file's st_nlink is
greater 1. So only for files that actually have several
links pointing to them hardlinks will be created, which
is what GNU tar does.

Will backport.