Revert "xdp: remove the double UMEM ring sizes, which aren't needed"

After the change the following assert sometimes occured when running
kxdpgun in the emulated XDP mode.

knot_xdp_recv_finish():
  const uint32_t reserved = xsk_ring_prod__reserve(fq, count, &idx);
  assert(reserved == count);

This reverts commit f5f4ee9f.