cpuset: separate tmpmasks and cpuset allocation logic

cpuset: separate tmpmasks and cpuset allocation logic

The original alloc_cpumasks() served dual purposes: allocating cpumasks
for both temporary masks (tmpmasks) and cpuset structures. This patch:

1. Decouples these allocation paths for better code clarity
2. Introduces dedicated alloc_tmpmasks() and dup_or_alloc_cpuset()
   functions
3. Maintains symmetric pairing:
   - alloc_tmpmasks() ↔ free_tmpmasks()
   - dup_or_alloc_cpuset() ↔ free_cpuset()

Signed-off-by: Chen Ridong <chenridong@huawei.com>
Reviewed-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>