xdr_set_scratch_buffer
xdr_set_scratch_buffer(xdr, res->scratch, READ_PLUS_SCRATCH_SIZE);
xdr_set_scratch_buffer(xdr, folio_address(folio), folio_size(folio));
xdr_set_scratch_buffer(xdr, NULL, 0);
xdr_set_scratch_buffer(xdr, xdr->p, frag1bytes);