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: Eric W. Biederman <ebiederm@xmission.com>

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;
 	};