[portsyncd]: Fix portsyncd restart case (sonic-net#1019)

I noticed that after swss restart in VS orchagent does not receive
PortInitDone. I looked at the comment about "g_portSet" says that:

 When this LinkSync class is
 * initialized, we check the database to see if some of the ports' host
 * interfaces are already created and remove them from this set.
However g_portSet was filled after LinkSync is initialized, so I
considered this is a bug causing orchagent does not receive PortInitDone
when portsyncd starts after host interfaces were created.

Signed-off-by: Stepan Blyschak <stepanb@mellanox.com>

portsyncd/linksync.cpp

@@ -95,20 +95,27 @@ LinkSync::LinkSync(DBConnector *appl_db, DBConnector *state_db) :
     if (!WarmStart::isWarmStart())
     {
         /* See the comments for g_portSet in portsyncd.cpp */
-        for (string port : g_portSet)
+        for (auto port_iter = g_portSet.begin(); port_iter != g_portSet.end();)
         {
+            string port = *port_iter;
             vector<FieldValueTuple> temp;
+            bool portFound = false;
             if (m_portTable.get(port, temp))
             {
                 for (auto it : temp)
                 {
                     if (fvField(it) == "admin_status")
                     {
-                        g_portSet.erase(port);
+                        port_iter = g_portSet.erase(port_iter);
+                        portFound = true;
                         break;
                     }
                 }
             }
+            if (!portFound)
+            {
+                ++port_iter;
+            }
         }
 
         for (idx_p = if_ni;

portsyncd/portsyncd.cpp

@@ -79,10 +79,6 @@ int main(int argc, char **argv)
     WarmStart::checkWarmStart("portsyncd", "swss");
     const bool warm = WarmStart::isWarmStart();
 
-    LinkSync sync(&appl_db, &state_db);
-    NetDispatcher::getInstance().registerMessageHandler(RTM_NEWLINK, &sync);
-    NetDispatcher::getInstance().registerMessageHandler(RTM_DELLINK, &sync);
-
     try
     {
         NetLink netlink;
@@ -102,6 +98,10 @@ int main(int argc, char **argv)
             }
         }
 
+        LinkSync sync(&appl_db, &state_db);
+        NetDispatcher::getInstance().registerMessageHandler(RTM_NEWLINK, &sync);
+        NetDispatcher::getInstance().registerMessageHandler(RTM_DELLINK, &sync);
+
         s.addSelectable(&netlink);
         s.addSelectable(&portCfg);