genirq: Fix export of irq_to_desc() for powerpc KVM

Commit 64a1b95b ("genirq: Restrict export of irq_to_desc()") removed
the export of irq_to_desc() unless powerpc KVM is being built, because
there is still a use of irq_to_desc() in modular code there.

However it used:

  #ifdef CONFIG_KVM_BOOK3S_64_HV

Which doesn't work when that symbol is =m, leading to a build failure:

  ERROR: modpost: "irq_to_desc" [arch/powerpc/kvm/kvm-hv.ko] undefined!

Fix it by checking for the definedness of the correct symbol which is
CONFIG_KVM_BOOK3S_64_HV_MODULE.

Fixes: 64a1b95b ("genirq: Restrict export of irq_to_desc()")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/irq/irqdesc.c

@@ -352,7 +352,7 @@ struct irq_desc *irq_to_desc(unsigned int irq)
 {
 	return radix_tree_lookup(&irq_desc_tree, irq);
 }
-#ifdef CONFIG_KVM_BOOK3S_64_HV
+#ifdef CONFIG_KVM_BOOK3S_64_HV_MODULE
 EXPORT_SYMBOL_GPL(irq_to_desc);
 #endif