docs: update HOWTO for 2.6.x -> 3.x versioning

commit 591bfc6bf9e5e25e464fd4c87d64afd5135667c4 upstream.

The HOWTO document needed updating for the new kernel versioning. The
git URI for -next was updated as well.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>