mm: mass constification of folio/page pointers

Now that page_pgoff() takes const pointers, we can constify the pointers
to a lot of functions.

Link: https://lkml.kernel.org/r/20241005200121.3231142-5-willy@infradead.org


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