fuse: revert back to __readahead_folio() for readahead

In the Linux kernel, the following vulnerability has been resolved:

fuse: revert back to __readahead_folio() for readahead

In commit 3eab9d7bc2f4 ("fuse: convert readahead to use folios"), the

logic was converted to using the new folio readahead code, which drops

the reference on the folio once it is locked, using an inferred

reference on the folio. Previously we held a reference on the folio for

the entire duration of the readpages call.

This is fine, however for the case for splice pipe responses where we

will remove the old folio and splice in the new folio (see

fuse_try_move_page()), we assume that there is a reference held on the

folio for ap->folios, which is no longer the case.

To fix this, revert back to __readahead_folio() which allows us to hold

the reference on the folio for the duration of readpages until either we

drop the reference ourselves in fuse_readpages_end() or the reference is

dropped after it's replaced in the page cache in the splice case.

This will fix the UAF bug that was reported.