fuse: convert readahead to use folios

fuse: convert readahead to use folios

Currently we're using the __readahead_batch() helper which populates our
fuse_args_pages->pages array with pages.  Convert this to use the newer
folio based pattern which is to call readahead_folio() to get the next
folio in the read ahead batch.  I've updated the code to use things like
folio_size() and to take into account larger folio sizes, but this is
purely to make that eventual work easier to do, we currently will not
get large folios so this is more future proofing than actual support.

[SzM: remove check for readahead_folio() won't return NULL (at least for
now) so remove ugly assign in conditional.]

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>