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

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>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>