Impact: build fix on ia64

ia64's default_affinity_write() still had old cpumask_t usage:

 /home/mingo/tip/kernel/irq/proc.c: In function `default_affinity_write':
 /home/mingo/tip/kernel/irq/proc.c:114: error: incompatible type for argument 1 of `is_affinity_mask_valid'
 make[3]: *** [kernel/irq/proc.o] Error 1
 make[3]: *** Waiting for unfinished jobs....

update it to cpumask_var_t.
Signed-off-by: Ingo Molnar <mingo@elte.hu>