tcp: keep snd sack block free list

Instead of constantly reallocating the new sack block list, keep the old
one as a reusable free list.

Change-Id: Iad79a72204f97b96352c1c6eea66c2839a35cfe6
Signed-off-by: Florin Coras <fcoras@cisco.com>
diff --git a/src/vnet/tcp/tcp_input.c b/src/vnet/tcp/tcp_input.c
index dbe1fc4..df8f53d 100644
--- a/src/vnet/tcp/tcp_input.c
+++ b/src/vnet/tcp/tcp_input.c
@@ -1714,7 +1714,7 @@
 void
 tcp_update_sack_list (tcp_connection_t * tc, u32 start, u32 end)
 {
-  sack_block_t *new_list = 0, *block = 0;
+  sack_block_t *new_list = tc->snd_sacks_fl, *block = 0;
   int i;
 
   /* If the first segment is ooo add it to the list. Last write might've moved
@@ -1758,7 +1758,8 @@
   ASSERT (vec_len (new_list) <= TCP_MAX_SACK_BLOCKS);
 
   /* Replace old vector with new one */
-  vec_free (tc->snd_sacks);
+  vec_reset_length (tc->snd_sacks);
+  tc->snd_sacks_fl = tc->snd_sacks;
   tc->snd_sacks = new_list;
 
   /* Segments should not 'touch' */