x86/apic: Export x2apic_mode for use by KVM in "warm" path

Export x2apic_mode so that KVM can query whether x2APIC is active
without having to incur the RDMSR in x2apic_enabled().  When Posted
Interrupts are in use for a guest with an assigned device, KVM ends up
checking for x2APIC at least once every time a vCPU halts.  KVM could
obviously snapshot x2apic_enabled() to avoid the RDMSR, but that's
rather silly given that x2apic_mode holds the exact info needed by KVM.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210115220354.434807-2-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kernel/apic/apic.c

@@ -1743,6 +1743,7 @@ void apic_ap_setup(void)
 
 #ifdef CONFIG_X86_X2APIC
 int x2apic_mode;
+EXPORT_SYMBOL_GPL(x2apic_mode);
 
 enum {
 	X2APIC_OFF,