mm: drop page_index and simplify folio_index

There are two helpers for retrieving the index within address space for
mixed usage of swap cache and page cache:

- page_index
- folio_index

This commit drops page_index, as we have eliminated all users, and
converts folio_index's helper __page_file_index to use folio to avoid the
page conversion.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Kairui Song <[email protected]>
Reviewed-by: "Huang, Ying" <[email protected]>
Cc: Anna Schumaker <[email protected]>
Cc: Barry Song <[email protected]>
Cc: Chao Yu <[email protected]>
Cc: Chris Li <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: David Howells <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Ilya Dryomov <[email protected]>
Cc: Jaegeuk Kim <[email protected]>
Cc: Jeff Layton <[email protected]>
Cc: Marc Dionne <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: NeilBrown <[email protected]>
Cc: Ryan Roberts <[email protected]>
Cc: Ryusuke Konishi <[email protected]>
Cc: Trond Myklebust <[email protected]>
Cc: Xiubo Li <[email protected]>
Cc: Yosry Ahmed <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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