[linux] Validate length of ACPI table read from sysfs

[linux] Validate length of ACPI table read from sysfs

Consumers of acpi_find() will assume that returned structures include
a valid table header and that the length in the table header is
correct.  These assumptions are necessary when dealing with raw ACPI
tables, since there exists no independent source of length
information.

Ensure that these assumptions are also valid for ACPI tables read from
sysfs.

Signed-off-by: Michael Brown <mcb30@ipxe.org>