samples/bpf: Fix to xdpsock to avoid recycling frames

The txpush program in the xdpsock sample application is supposed
to send out all packets in the umem in a round-robin fashion.
The problem is that it only cycled through the first BATCH_SIZE
worth of packets. Fixed this so that it cycles through all buffers
in the umem as intended.

Fixes: 248c7f9c ("samples/bpf: convert xdpsock to use libbpf for AF_XDP access")
Signed-off-by: Weqaar Janjua <weqaar.a.janjua@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Björn Töpel <bjorn.topel@intel.com>
Link: https://lore.kernel.org/bpf/20200828161717.42705-1-weqaar.a.janjua@intel.com

samples/bpf/xdpsock_user.c

@@ -1010,7 +1010,7 @@ static void rx_drop_all(void)
 	}
 }
 
-static void tx_only(struct xsk_socket_info *xsk, u32 frame_nb, int batch_size)
+static void tx_only(struct xsk_socket_info *xsk, u32 *frame_nb, int batch_size)
 {
 	u32 idx;
 	unsigned int i;
@@ -1023,14 +1023,14 @@ static void tx_only(struct xsk_socket_info *xsk, u32 frame_nb, int batch_size)
 	for (i = 0; i < batch_size; i++) {
 		struct xdp_desc *tx_desc = xsk_ring_prod__tx_desc(&xsk->tx,
 								  idx + i);
-		tx_desc->addr = (frame_nb + i) << XSK_UMEM__DEFAULT_FRAME_SHIFT;
+		tx_desc->addr = (*frame_nb + i) << XSK_UMEM__DEFAULT_FRAME_SHIFT;
 		tx_desc->len = PKT_SIZE;
 	}
 
 	xsk_ring_prod__submit(&xsk->tx, batch_size);
 	xsk->outstanding_tx += batch_size;
-	frame_nb += batch_size;
-	frame_nb %= NUM_FRAMES;
+	*frame_nb += batch_size;
+	*frame_nb %= NUM_FRAMES;
 	complete_tx_only(xsk, batch_size);
 }
 
@@ -1086,7 +1086,7 @@ static void tx_only_all(void)
 		}
 
 		for (i = 0; i < num_socks; i++)
-			tx_only(xsks[i], frame_nb[i], batch_size);
+			tx_only(xsks[i], &frame_nb[i], batch_size);
 
 		pkt_cnt += batch_size;