EDAC, thunderx: Remove VLA usage

In the quest to remove all stack VLA usage from the kernel[1], switch to
using a kmalloc-allocated buffer instead of stack space. This should be
fine since the existing routine is allocating memory too.

Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Jan Glauber <jglauber@cavium.com>
Cc: David Daney <david.daney@cavium.com>
Cc: linux-edac <linux-edac@vger.kernel.org>
Link: http://lkml.kernel.org/r/20180629184850.GA37464@beast
Link: https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@mail.gmail.com

 [1]
Signed-off-by: Borislav Petkov <bp@suse.de>