KVM: MMU: make kvm_mmu_available_pages robust against n_used_mmu_pages > n_max_mmu_pages

[Marcelo Tosatti <mtosatti@redhat.com>]

As noticed by Ulrich Obergfell <uobergfe@redhat.com>, the mmu
counters are for beancounting purposes only - so n_used_mmu_pages and
n_max_mmu_pages could be relaxed (example: before f0f5933a1626c8df7b),
resulting in n_used_mmu_pages > n_max_mmu_pages.

Make code robust against n_used_mmu_pages > n_max_mmu_pages.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
index 6987108..3b1ad00 100644
--- a/arch/x86/kvm/mmu.h
+++ b/arch/x86/kvm/mmu.h
@@ -57,8 +57,11 @@ int kvm_init_shadow_mmu(struct kvm_vcpu *vcpu, struct kvm_mmu *context);
 
 static inline unsigned int kvm_mmu_available_pages(struct kvm *kvm)
 {
-	return kvm->arch.n_max_mmu_pages -
-		kvm->arch.n_used_mmu_pages;
+	if (kvm->arch.n_max_mmu_pages > kvm->arch.n_used_mmu_pages)
+		return kvm->arch.n_max_mmu_pages -
+			kvm->arch.n_used_mmu_pages;
+
+	return 0;
 }
 
 static inline void kvm_mmu_free_some_pages(struct kvm_vcpu *vcpu)