gfs2: increase usage of folio_next_index() helper

Simplify code pattern of 'folio->index + folio_nr_pages(folio)' by using
the existing helper folio_next_index().

Signed-off-by: Minjie Du <duminjie@vivo.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>