KVM: TDX: Avoid a double-KVM_BUG_ON() in tdx_sept_zap_private_spte()

Return -EIO immediately from tdx_sept_zap_private_spte() if the number of
to-be-added pages underflows, so that the following "KVM_BUG_ON(err, kvm)"
isn't also triggered.  Isolating the check from the "is premap error"
if-statement will also allow adding a lockdep assertion that premap errors
are encountered if and only if slots_lock is held.

Reviewed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: Kai Huang <kai.huang@intel.com>
Reviewed-by: Yan Zhao <yan.y.zhao@intel.com>
Tested-by: Yan Zhao <yan.y.zhao@intel.com>
Tested-by: Kai Huang <kai.huang@intel.com>
Link: https://patch.msgid.link/20251030200951.3402865-14-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/vmx/tdx.c

@@ -1725,8 +1725,10 @@ static int tdx_sept_zap_private_spte(struct kvm *kvm, gfn_t gfn,
 		err = tdh_mem_range_block(&kvm_tdx->td, gpa, tdx_level, &entry, &level_state);
 		tdx_no_vcpus_enter_stop(kvm);
 	}
-	if (tdx_is_sept_zap_err_due_to_premap(kvm_tdx, err, entry, level) &&
-	    !KVM_BUG_ON(!atomic64_read(&kvm_tdx->nr_premapped), kvm)) {
+	if (tdx_is_sept_zap_err_due_to_premap(kvm_tdx, err, entry, level)) {
+		if (KVM_BUG_ON(!atomic64_read(&kvm_tdx->nr_premapped), kvm))
+			return -EIO;
+
 		atomic64_dec(&kvm_tdx->nr_premapped);
 		return 0;
 	}