cbtable.c: don't assume high addresses can fully map 1 MiB

Forward port the downstream `commit b17e9e41838`.

When using a forwarding table entry for finding the coreboot table
don't assume one has access to a full 1 MiB where the forwarding
table entry points to. The reason is that the 1 MiB may cover address
regions that have differing cacheability type. As such the kernel will
complain and the mapping will fail. Instead, check the header first then
map in the bytes that it indicates after sanity validation. That way
there is no attempt at requesting an invalid mapping that spans different
memory cacheability attributes.

V.2: Incorperate Nico's and Angels comments from upstream.

BUG=b:66681446
BRANCH=None
TEST=Can successfully run 'flashrom -p host --wp-status' on kahlee
     without generating PAT errors.

Original-Change-Id: Ic6c5832b069300cced66e11f4ca4a0bbc6e496de
Original-Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Original-Reviewed-on: https://chromium-review.googlesource.com/685608
Original-Reviewed-by: Martin Roth <martinroth@chromium.org>
Original-Reviewed-by: Justin TerAvest <teravest@chromium.org>

Change-Id: I43705c19dd7c816098d03f528bde6f180c4c8f24
Signed-off-by: Edward O'Callaghan <quasisec@chromium.org>
---
M cbtable.c
1 file changed, 60 insertions(+), 8 deletions(-)