[routesync] Fix for stale dynamic neighbor

Signed-off-by: vedganes <veda.ganesan@nokia.com>

The changes are for fixing stale neighbor in the ASIC_DB and data path
when eBGP neighbors are shutdown and neighbors are flushed. The problem
is described in issue: sonic-net/sonic-buildimage#12442
The root cause of this issue is due to not deleing the route from the
ASIC_DB when the route's next hop is on eth0 or docker0 interface. The solution is
to delete the route entry from ASIC_DB instead of just returning when the route's
next hop is on the interface eth0 or docker0

Author: vganesan-nokia

fpmsyncd/routesync.cpp

@@ -733,6 +733,17 @@ void RouteSync::onRouteMsg(int nlmsg_type, struct nl_object *obj, char *vrf)
         {
             SWSS_LOG_DEBUG("Skip routes to eth0 or docker0: %s %s %s",
                     destipprefix, gw_list.c_str(), intf_list.c_str());
+            // If intf_list has only this interface, that means all of the next hops of this route 
+            // have been removed and the next hop on the eth0/docker0 has become the only next hop. 
+            // In this case since we do not want the route with next hop on eth0/docker0, we return. 
+            // But still we need to clear the route from the APPL_DB. Otherwise the APPL_DB and data 
+            // path will be left with stale route entry
+            if(alsv.size() == 1)
+            {
+                SWSS_LOG_NOTICE("RouteTable del msg for route with only one nh on eth0/docker0: %s %s %s %s",
+                        destipprefix, gw_list.c_str(), intf_list.c_str(), mpls_list.c_str());
+                m_routeTable.del(destipprefix);
+            }
             return;
         }
     }