kbuild: deb-pkg: make .orig tarball a hard link if possible

If '..' belongs to the same filesystem, create a hard link instead of
a copy. In most cases, you can save disk space.

I do not want to use 'mv' because keeping linux.tar.gz is useful to
avoid unneeded rebuilding of the tarball.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>