KVM: x86/mmu: avoid NULL-pointer dereference on page freeing bugs

WARN and bail if KVM attempts to free a root that isn't backed by a shadow
page.  KVM allocates a bare page for "special" roots, e.g. when using PAE
paging or shadowing 2/3/4-level page tables with 4/5-level, and so root_hpa
will be valid but won't be backed by a shadow page.  It's all too easy to
blindly call mmu_free_root_page() on root_hpa, be nice and WARN instead of
crashing KVM and possibly the kernel.
Reviewed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/mmu.c

@@ -3206,6 +3206,8 @@ static void mmu_free_root_page(struct kvm *kvm, hpa_t *root_hpa,
 		return;
 
 	sp = to_shadow_page(*root_hpa & PT64_BASE_ADDR_MASK);
+	if (WARN_ON(!sp))
+		return;
 
 	if (is_tdp_mmu_page(sp))
 		kvm_tdp_mmu_put_root(kvm, sp, false);