mm, slab: fix the comment of cpu partial list

The partial slabs on cpu partial list are not frozen after the commit
8cd3fa42

 ("slub: Delay freezing of partial slabs") merged. So fix
the comment.

Signed-off-by: Chengming Zhou <chengming.zhou@linux.dev>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>