niu: Use page->private instead of page->index

We are close to removing page->index.  Use page->private instead, which
is least likely to be removed.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Link: https://patch.msgid.link/20241216155124.3114-1-willy@infradead.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>