mm: use page->private instead of page->index in percpu

The percpu allocator only uses one field in struct page, just change it
from page->index to page->private.

Link: https://lkml.kernel.org/r/20241005200121.3231142-8-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>