null_blk: Remove accesses to page->index

Use page->private to store the index instead of page->index.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Link: https://lore.kernel.org/r/20241216160849.31739-1-willy@infradead.org


Signed-off-by: Jens Axboe <axboe@kernel.dk>