NFS: Convert the readdir array-of-pages into an array-of-folios

This patch only converts the actual array, but doesn't touch the
individual nfs_cache_array pages and related functions (that will be
done in the next patch).

I also adjust the names of the fields in the nfs_readdir_descriptor to
say "folio" instead of "page".
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>