ceph: Remove uses of page from ceph_process_folio_batch()

ceph: Remove uses of page from ceph_process_folio_batch()

Remove uses of page->index and deprecated page APIs.  Saves a lot of
hidden calls to compound_head().

Also convert is_page_index_contiguous() to is_folio_index_contiguous()
and make its arguments const.

Signed-off-by: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Link: https://lore.kernel.org/r/20250217185119.430193-8-willy@infradead.org
Tested-by: Viacheslav Dubeyko <Slava.Dubeyko@ibm.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>