x86/apic: enable x2APIC mode before doing any setup

x86/apic: enable x2APIC mode before doing any setup

Current code calls apic_x2apic_probe which does some initialization
and setup before having enabled x2APIC mode (if it's not already
enabled by the firmware).

This can lead to issues if the APIC ID doesn't match the x2APIC ID, as
apic_x2apic_probe calls init_apic_ldr_x2apic_cluster which depending
on the APIC mode might set cpu_2_logical_apicid using the APIC ID
instead of the x2APIC ID (because x2APIC might not be enabled yet).

Fix this by enabling x2APIC before calling apic_x2apic_probe.

As a remark, this was discovered while I was trying to figure out why
one of my test boxes didn't report any iommu faults. The root cause
was that the iommu MSI address field was set using the stale value in
cpu_2_logical_apicid, and thus the iommu fault interrupt would get
lost. Even if the MSI address field gets sets to a correct value
afterwards as soon as a single iommu fault is pending no further
interrupts would get injected, so losing a single iommu fault
interrupt is fatal.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>