mm/percpu,c: remove obsolete comments of pcpu_chunk_populated()

Commit b239f7da ("percpu: set PCPU_BITMAP_BLOCK_SIZE to PAGE_SIZE")
removed the parameter 'for_alloc', so remove this comment.

Link: https://lkml.kernel.org/r/1630576043-21367-1-git-send-email-jingxiangfeng@huawei.com


Signed-off-by: Jing Xiangfeng <jingxiangfeng@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>