linux: Fix URL for release candidate versions

Starting with 4.12-rc1, tarballs are generated by cgit directly from
Linus's tree. This also implies that no .tar.xz can be used for them.

This method also applies to older release candidates.

Signed-off-by: Luis Araneda <luaraneda@gmail.com>
[Arnout: added comment in the code]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>