ACPI: fix APEI related table size checking

On Huang Ying's machine:

erst_tab->header_length == sizeof(struct acpi_table_einj)

but Yinghai reported that on his machine,

erst_tab->header_length == sizeof(struct acpi_table_einj) -
sizeof(struct acpi_table_header)

To make erst table size checking code works on all systems, both
testing are treated as PASS.

Same situation applies to einj_tab->header_length, so corresponding
table size checking is changed in similar way too.

Originally-by: Yinghai Lu <yinghai@xxxxxxxxxx>
Signed-off-by: Huang Ying <ying.huang@xxxxxxxxx>

- use switch() for better readability
- add comment explaining why a formally invalid size it also being
  accepted
- check erst_tab->header.length before even looking at
  erst_tab->header_length
- prefer sizeof(*erst_tab) over sizeof(struct acpi_table_erst)

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/drivers/acpi/apei/erst.c
+++ b/xen/drivers/acpi/apei/erst.c
@@ -715,12 +715,23 @@ int erst_clear(u64 record_id)
 
 static int __init erst_check_table(struct acpi_table_erst *erst_tab)
 {
-	if (erst_tab->header_length != sizeof(struct acpi_table_erst))
+	if (erst_tab->header.length < sizeof(*erst_tab))
 		return -EINVAL;
-	if (erst_tab->header.length < sizeof(struct acpi_table_erst))
+
+	switch (erst_tab->header_length) {
+	case sizeof(*erst_tab) - sizeof(erst_tab->header):
+	/*
+	 * While invalid per specification, there are (early?) systems
+	 * indicating the full header size here, so accept that value too.
+	 */
+	case sizeof(*erst_tab):
+		break;
+	default:
 		return -EINVAL;
+	}
+
 	if (erst_tab->entries !=
-	    (erst_tab->header.length - sizeof(struct acpi_table_erst)) /
+	    (erst_tab->header.length - sizeof(*erst_tab)) /
 	    sizeof(struct acpi_erst_entry))
 		return -EINVAL;