x86/acpi: Prevent X2APIC id 0xffffffff from being accounted

RongQing reported that there are some X2APIC id 0xffffffff in his machine's
ACPI MADT table, which makes the number of possible CPU inaccurate.

The reason is that the ACPI X2APIC parser has no sanity check for APIC ID
0xffffffff, which is an invalid id in all APIC types. See "Intel® 64
Architecture x2APIC Specification", Chapter 2.4.1.

Add a sanity check to acpi_parse_x2apic() which ignores the invalid id.

Reported-by: Li RongQing <lirongqing@baidu.com>
Signed-off-by: Dou Liyang <douly.fnst@cn.fujitsu.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Cc: len.brown@intel.com
Cc: rjw@rjwysocki.net
Cc: hpa@zytor.com
Link: https://lkml.kernel.org/r/20180412014052.25186-1-douly.fnst@cn.fujitsu.com

arch/x86/kernel/acpi/boot.c

@@ -215,6 +215,10 @@ acpi_parse_x2apic(struct acpi_subtable_header *header, const unsigned long end)
 	apic_id = processor->local_apic_id;
 	enabled = processor->lapic_flags & ACPI_MADT_ENABLED;
 
+	/* Ignore invalid ID */
+	if (apic_id == 0xffffffff)
+		return 0;
+
 	/*
 	 * We need to register disabled CPU as well to permit
 	 * counting disabled CPUs. This allows us to size