selftests/xsk: Fix for SEND_RECEIVE_UNALIGNED test

Fix test broken by shared umem test and framework enhancement commit.

Correct the current implementation of pkt_stream_replace_half() by
ensuring that nb_valid_entries are not set to half, as this is not true
for all the tests. Ensure that the expected value for valid_entries for
the SEND_RECEIVE_UNALIGNED test equals the total number of packets sent,
which is 4096.

Create a new function called pkt_stream_pkt_set() that allows for packet
modification to meet specific requirements while ensuring the accurate
maintenance of the valid packet count to prevent inconsistencies in packet
tracking.

Fixes: 6d198a89 ("selftests/xsk: Add a test for shared umem feature")
Reported-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Tushar Vyavahare <tushar.vyavahare@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Acked-by: Magnus Karlsson <magnus.karlsson@intel.com>
Link: https://lore.kernel.org/bpf/20231214130007.33281-1-tushar.vyavahare@intel.com

tools/testing/selftests/bpf/xskxceiver.c

@@ -634,16 +634,24 @@ static u32 pkt_nb_frags(u32 frame_size, struct pkt_stream *pkt_stream, struct pk
 	return nb_frags;
 }
 
+static bool set_pkt_valid(int offset, u32 len)
+{
+	return len <= MAX_ETH_JUMBO_SIZE;
+}
+
 static void pkt_set(struct pkt_stream *pkt_stream, struct pkt *pkt, int offset, u32 len)
 {
 	pkt->offset = offset;
 	pkt->len = len;
-	if (len > MAX_ETH_JUMBO_SIZE) {
-		pkt->valid = false;
-	} else {
-		pkt->valid = true;
-		pkt_stream->nb_valid_entries++;
-	}
+	pkt->valid = set_pkt_valid(offset, len);
+}
+
+static void pkt_stream_pkt_set(struct pkt_stream *pkt_stream, struct pkt *pkt, int offset, u32 len)
+{
+	bool prev_pkt_valid = pkt->valid;
+
+	pkt_set(pkt_stream, pkt, offset, len);
+	pkt_stream->nb_valid_entries += pkt->valid - prev_pkt_valid;
 }
 
 static u32 pkt_get_buffer_len(struct xsk_umem_info *umem, u32 len)
@@ -665,7 +673,7 @@ static struct pkt_stream *__pkt_stream_generate(u32 nb_pkts, u32 pkt_len, u32 nb
 	for (i = 0; i < nb_pkts; i++) {
 		struct pkt *pkt = &pkt_stream->pkts[i];
 
-		pkt_set(pkt_stream, pkt, 0, pkt_len);
+		pkt_stream_pkt_set(pkt_stream, pkt, 0, pkt_len);
 		pkt->pkt_nb = nb_start + i * nb_off;
 	}
 
@@ -700,10 +708,9 @@ static void __pkt_stream_replace_half(struct ifobject *ifobj, u32 pkt_len,
 
 	pkt_stream = pkt_stream_clone(ifobj->xsk->pkt_stream);
 	for (i = 1; i < ifobj->xsk->pkt_stream->nb_pkts; i += 2)
-		pkt_set(pkt_stream, &pkt_stream->pkts[i], offset, pkt_len);
+		pkt_stream_pkt_set(pkt_stream, &pkt_stream->pkts[i], offset, pkt_len);
 
 	ifobj->xsk->pkt_stream = pkt_stream;
-	pkt_stream->nb_valid_entries /= 2;
 }
 
 static void pkt_stream_replace_half(struct test_spec *test, u32 pkt_len, int offset)