KVM: Call kvm_arch_flush_shadow_memslot() on the old slot in kvm_invalidate_memslot()

kvm_invalidate_memslot() calls kvm_arch_flush_shadow_memslot() on the
active, but KVM_MEMSLOT_INVALID slot.
Do it on the inactive (but valid) old slot instead since arch code really
should not get passed such invalid slot.

Note that this means that the "arch" field of the slot provided to
kvm_arch_flush_shadow_memslot() may have stale data since this function
is called with slots_arch_lock released.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Message-Id: <813595ecc193d6ae39a87709899d4251523b05f8.1638817641.git.maciej.szmigiero@oracle.com>

virt/kvm/kvm_main.c

@@ -1630,7 +1630,7 @@ static void kvm_invalidate_memslot(struct kvm *kvm,
 	 *	- gfn_to_hva (kvm_read_guest, gfn_to_pfn)
 	 *	- kvm_is_visible_gfn (mmu_check_root)
 	 */
-	kvm_arch_flush_shadow_memslot(kvm, working_slot);
+	kvm_arch_flush_shadow_memslot(kvm, old);
 
 	/* Was released by kvm_swap_active_memslots, reacquire. */
 	mutex_lock(&kvm->slots_arch_lock);