kern/acpi: Fix out of bounds access in grub_acpi_xsdt_find_table()

kern/acpi: Fix out of bounds access in grub_acpi_xsdt_find_table()

The calculation of the size of the table was incorrect (copy/pasta from
grub_acpi_rsdt_find_table() I assume...). The entries are 64-bit long.

This causes us to access beyond the end of the table which is causing
crashes during boot on some systems. Typically this is causing a crash
on VMWare when using UEFI and enabling serial autodetection, as

  grub_acpi_find_table (GRUB_ACPI_SPCR_SIGNATURE);

will goes past the end of the table (the SPCR table doesn't exits).

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Reviewed-by: Vladimir Serbinenko <phcoder@gmail.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
Reviewed-by: Ross Philipson <ross.philipson@oracle.com>
Tested-by: Renata Ravanelli <rravanel@redhat.com>