Agent crash @ FlowMgmtEntry::Delete

Problem:
Note that Flow management module and controller update module run in parallel.
In BGP service flows, to identify flow mgmt tree, CN info is needed.
When a bgp service flow is getting added keys are extracted and then flow is
added. Both these routines use GetCNIndex to identify which bgp service flow
mgmt tree to use. This can cause issue as extraction of keys may have taken some
index but by the time flow node gets added in tree CN which has been picked up
is down. In turn GetCNIndex will return invalid index and flow node does not get
added. Because of CN delete, flow delete request would have been pushed. On
serving delete request key is found but flow node is absent. This results in
crash.

Solution:
During addition of flow node use CN index from key instead of calling
GetCNIndex.

Change-Id: Ifd9adac9cdb4a03e621cf62eff009e3e55d882a5
Closes-bug: #1630011
(cherry picked from commit ea23d21)

src/vnsw/agent/pkt/flow_mgmt.cc

@@ -790,7 +790,9 @@ void FlowMgmtManager::AddFlowMgmtKey(FlowEntry *flow, FlowEntryInfo *info,
         break;
 
     case FlowMgmtKey::BGPASASERVICE: {
-        int cn_index = BgpAsAServiceFlowMgmtTree::GetCNIndex(flow);
+        BgpAsAServiceFlowMgmtKey *bgp_service_key =
+            static_cast<BgpAsAServiceFlowMgmtKey *>(key);
+        int cn_index = bgp_service_key->cn_index();
         if (cn_index != BgpAsAServiceFlowMgmtTree::kInvalidCnIndex) {
             bgp_as_a_service_flow_mgmt_tree_[cn_index].get()->Add(key, flow,
                                                   (ret.second)? node : NULL);