Add downstream fix for UDP buffer chaining

Upstream CR: https://gerrit.fd.io/r/c/vpp/+/31647 (merged)
vpp-dev thread: https://lists.fd.io/g/vpp-dev/message/18921

vpp-patches/0013-session-fix-populating-chained-buffers.patch

@@ -0,0 +1,42 @@
+From bbee13b6f04515ee42f066f3f0bfc6b675b5a9a3 Mon Sep 17 00:00:00 2001
+From: Ivan Shvedunov <[email protected]>
+Date: Mon, 15 Mar 2021 19:05:14 +0300
+Subject: [PATCH] session: fix populating chained buffers
+
+Type: fix
+
+Signed-off-by: Ivan Shvedunov <[email protected]>
+Change-Id: I3fdc1711d508bc5b0c2f56b3bd3fb4a56056eb54
+---
+ src/vnet/session/session_node.c | 8 +++++---
+ 1 file changed, 5 insertions(+), 3 deletions(-)
+
+diff --git a/src/vnet/session/session_node.c b/src/vnet/session/session_node.c
+index 2fde85b03..0add12b97 100644
+--- a/src/vnet/session/session_node.c
++++ b/src/vnet/session/session_node.c
+@@ -630,16 +630,18 @@ session_tx_fifo_chain_tail (vlib_main_t * vm, session_tx_context_t * ctx,
+ 	      svm_fifo_t *f = ctx->s->tx_fifo;
+ 	      session_dgram_hdr_t *hdr = &ctx->hdr;
+ 	      u16 deq_now;
++	      u32 offset;
++
+ 	      deq_now = clib_min (hdr->data_length - hdr->data_offset,
+ 				  len_to_deq);
+-	      n_bytes_read = svm_fifo_peek (f, hdr->data_offset, deq_now,
+-					    data);
++	      offset = hdr->data_offset + SESSION_CONN_HDR_LEN;
++	      n_bytes_read = svm_fifo_peek (f, offset, deq_now, data);
+ 	      ASSERT (n_bytes_read > 0);
+
+ 	      hdr->data_offset += n_bytes_read;
+ 	      if (hdr->data_offset == hdr->data_length)
+ 		{
+-		  u32 offset = hdr->data_length + SESSION_CONN_HDR_LEN;
++		  offset = hdr->data_length + SESSION_CONN_HDR_LEN;
+ 		  svm_fifo_dequeue_drop (f, offset);
+ 		  if (ctx->left_to_snd > n_bytes_read)
+ 		    svm_fifo_peek (ctx->s->tx_fifo, 0, sizeof (ctx->hdr),
+-- 
+2.30.2
+