x86, setup: guard against pre-ACPI 3 e820 code not updating %ecx

Impact: BIOS bug safety

For pre-ACPI 3 BIOSes, pre-initialize the end of the e820 buffer just
in case the BIOS returns an unchanged %ecx but without actually
touching the ACPI 3 extended flags field.
Signed-off-by: H. Peter Anvin <hpa@zytor.com>

arch/x86/boot/memory.c

@@ -31,6 +31,12 @@ static int detect_memory_e820(void)
 	struct e820entry *desc = boot_params.e820_map;
 	static struct e820_ext_entry buf; /* static so it is zeroed */
 
+	/*
+	 * Set this here so that if the BIOS doesn't change this field
+	 * but still doesn't change %ecx, we're still okay...
+	 */
+	buf.ext_flags = 1;
+
 	do {
 		size = sizeof buf;