cpuset: convert 'allowed' in __cpuset_node_allowed() to be boolean

Convert 'allowed' in __cpuset_node_allowed() to be boolean since the
return types of node_isset() and __cpuset_node_allowed() are both
boolean.

Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
Signed-off-by: Tejun Heo <tj@kernel.org>