kern/acpi: Skip NULL entries in RSDT and XSDT

kern/acpi: Skip NULL entries in RSDT and XSDT

During attempts to configure a serial console, a Page Fault Exception
and system reset were encountered, specifically on release 2.12~rc1.
This issue was not present in prior versions and seemed to affect only
a specific machine, potentially pointing to hardware or firmware flaw.

After investigation, it was discovered that the invalid page access
occurred during the discovery of serial MMIO ports as specified by
ACPI's SPCR table [1]. The recent change uncovered an issue in GRUB's
ACPI driver.

In certain cases, the XSDT/RSDT root table might contain a NULL entry as
a terminator, depending on how the tables are assembled. GRUB cannot
blindly trust the address in the root table to be valid and should
perform a sanity check for NULL entries. This patch introduces this
simple check.

This fix is also inspired by a related Linux kernel fix [2].

[1] 7b192ec4c term/ns8250: Use ACPI SPCR table when available to configure serial
[2] 0f929fbf0 ACPICA: Tables: Add new mechanism to skip NULL entries in RSDT and XSDT.

Signed-off-by: Michael Chang <mchang@suse.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>