selftests, xsk: Fix rx_full stats test

Fix the rx_full stats test so that it correctly reports pass even when
the fill ring is not full of buffers.

Fixes: 872a1184db ("selftests: xsk: Put the same buffer only once in the fill ring")
Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Acked-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Link: https://lore.kernel.org/bpf/20220121123508.12759-1-magnus.karlsson@gmail.com

tools/testing/selftests/bpf/xdpxceiver.c

@@ -906,7 +906,10 @@ static bool rx_stats_are_valid(struct ifobject *ifobject)
 			return true;
 		case STAT_TEST_RX_FULL:
 			xsk_stat = stats.rx_ring_full;
-			expected_stat -= RX_FULL_RXQSIZE;
+			if (ifobject->umem->num_frames < XSK_RING_PROD__DEFAULT_NUM_DESCS)
+				expected_stat = ifobject->umem->num_frames - RX_FULL_RXQSIZE;
+			else
+				expected_stat = XSK_RING_PROD__DEFAULT_NUM_DESCS - RX_FULL_RXQSIZE;
 			break;
 		case STAT_TEST_RX_FILL_EMPTY:
 			xsk_stat = stats.rx_fill_ring_empty_descs;