cpuset: clean up cpuset_node_allowed

cpuset: clean up cpuset_node_allowed

Commit 002f290627c2 ("cpuset: use static key better and convert to new
API") used __cpuset_node_allowed() instead of cpuset_node_allowed() to
check whether we can allocate on a memory node.  Now this function isn't
used by anyone, so we can do the follow things to clean it up.

1. remove unused codes
2. rename __cpuset_node_allowed() to cpuset_node_allowed()
3. update comments in mm/page_alloc.c

Link: https://lkml.kernel.org/r/20230228083537.102665-1-haifeng.xu@shopee.com
Signed-off-by: Haifeng Xu <haifeng.xu@shopee.com>
Suggested-by: Waiman Long <longman@redhat.com>
Acked-by: Waiman Long <longman@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Zefan Li <lizefan.x@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>