sysctl: Initial support for auto-unregistering sysctl tables.

Add nreg to ctl_table_header.  When nreg drops to 0 the ctl_table_header
will be unregistered.

Factor out drop_sysctl_table from unregister_sysctl_table, and add
the logic for decrementing nreg.
Signed-off-by: NEric W. Biederman <ebiederm@xmission.com>

fs/proc/proc_sysctl.c

@@ -29,6 +29,7 @@ static struct ctl_table root_table[1];
 static struct ctl_table_root sysctl_table_root;
 static struct ctl_table_header root_table_header = {
 	{{.count = 1,
+	  .nreg = 1,
 	  .ctl_table = root_table,
 	  .ctl_entry = LIST_HEAD_INIT(sysctl_table_root.default_set.list),}},
 	.root = &sysctl_table_root,
@@ -938,6 +939,7 @@ struct ctl_table_header *__register_sysctl_table(
 	header->unregistering = NULL;
 	header->root = root;
 	header->count = 1;
+	header->nreg = 1;
 	if (sysctl_check_table(path, table))
 		goto fail;
 
@@ -1192,6 +1194,20 @@ struct ctl_table_header *register_sysctl_table(struct ctl_table *table)
 }
 EXPORT_SYMBOL(register_sysctl_table);
 
+static void drop_sysctl_table(struct ctl_table_header *header)
+{
+	if (--header->nreg)
+		return;
+
+	start_unregistering(header);
+	if (!--header->parent->count) {
+		WARN_ON(1);
+		kfree_rcu(header->parent, rcu);
+	}
+	if (!--header->count)
+		kfree_rcu(header, rcu);
+}
+
 /**
  * unregister_sysctl_table - unregister a sysctl table hierarchy
  * @header: the header returned from register_sysctl_table
@@ -1224,13 +1240,7 @@ void unregister_sysctl_table(struct ctl_table_header * header)
 	}
 
 	spin_lock(&sysctl_lock);
-	start_unregistering(header);
-	if (!--header->parent->count) {
-		WARN_ON(1);
-		kfree_rcu(header->parent, rcu);
-	}
-	if (!--header->count)
-		kfree_rcu(header, rcu);
+	drop_sysctl_table(header);
 	spin_unlock(&sysctl_lock);
 }
 EXPORT_SYMBOL(unregister_sysctl_table);

include/linux/sysctl.h

@@ -1032,6 +1032,7 @@ struct ctl_table_header
 			struct list_head ctl_entry;
 			int used;
 			int count;
+			int nreg;
 		};
 		struct rcu_head rcu;
 	};