mm: remove references to page->index in huge_memory.c

We already have folios in all these places; it's just a matter of using
them instead of the pages.

Link: https://lkml.kernel.org/r/20241005200121.3231142-7-willy@infradead.org


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