cpumask: rename tsk_cpumask to tsk_cpus_allowed

Noone uses this wrapper yet, and Ingo asked that it be kept consistent
with current task_struct usage.

(One user crept in via linux-next: fixed)

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au.>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Tejun Heo <tj@kernel.org>