cpumask: add cpumask_var_t documentation

 *	  ... use 'tmpmask' like a normal struct cpumask * ...

 *

 *	free_cpumask_var(tmpmask);

 *

 *

 * However, one notable exception is there. alloc_cpumask_var() allocates

 * only nr_cpumask_bits bits (in the other hand, real cpumask_t always has

 * NR_CPUS bits). Therefore you don't have to dereference cpumask_var_t.

 *

 *	cpumask_var_t tmpmask;

 *	if (!alloc_cpumask_var(&tmpmask, GFP_KERNEL))

 *		return -ENOMEM;

 *

 *	var = *tmpmask;

 *

 * This code makes NR_CPUS length memcopy and brings to a memory corruption.

 * cpumask_copy() provide safe copy functionality.

 */

#ifdef CONFIG_CPUMASK_OFFSTACK

typedef struct cpumask *cpumask_var_t;