KVM: x86: Reduce refcount if single_open() fails in kvm_mmu_rmaps_stat_open()

Refcount is increased before calling single_open() in
kvm_mmu_rmaps_stat_open(), If single_open() fails, refcount should be
restored, otherwise the vm couldn't be destroyed.

Fixes: 3bcd0662 ("KVM: X86: Introduce mmu_rmaps_stat per-vm debugfs file")
Signed-off-by: Hou Wenlong <houwenlong.hwl@antgroup.com>
Message-Id: <a75900413bb8b1e556be690e9588a0f92e946a30.1665733883.git.houwenlong.hwl@antgroup.com>
[Preserved return value of single_open. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/debugfs.c

@@ -158,11 +158,16 @@ static int kvm_mmu_rmaps_stat_show(struct seq_file *m, void *v)
 static int kvm_mmu_rmaps_stat_open(struct inode *inode, struct file *file)
 {
 	struct kvm *kvm = inode->i_private;
+	int r;
 
 	if (!kvm_get_kvm_safe(kvm))
 		return -ENOENT;
 
-	return single_open(file, kvm_mmu_rmaps_stat_show, kvm);
+	r = single_open(file, kvm_mmu_rmaps_stat_show, kvm);
+	if (r < 0)
+		kvm_put_kvm(kvm);
+
+	return r;
 }
 
 static int kvm_mmu_rmaps_stat_release(struct inode *inode, struct file *file)