Avoid critical sections where possible

[htibosch]

Description

Recently, there was a request in #1058: "Please document how the synchronisation works".
This was about the socket's List_t that stores UDP packets, called xWaitingPacketsList.

I showed that the addition and removal of packets is protected by a critical section, and/or a suspended scheduler.
Non protected access is only seen when listCURRENT_LIST_LENGTH() is called. That is OK because a variable of type UBaseType_t is read.

These are the proposed changes:

In FreeRTOS_UDP_IPv4.c and in FreeRTOS_UDP_IPv6.c :

 vTaskSuspendAll();
 {
-    taskENTER_CRITICAL();
-    {
        /* Add the network packet to the list of packets to be
         * processed by the socket. */
        vListInsertEnd( &( pxSocket->u.xUDP.xWaitingPacketsList ), &( pxNetworkBuffer->xBufferListItem ) );
-    }
-    taskEXIT_CRITICAL();
 }
( void ) xTaskResumeAll();

And here in FreeRTOS_Sockets.c :

-    taskENTER_CRITICAL();
+    vTaskSuspendAll();
     {
         ( void ) uxListRemove( &( pxNetworkBuffer->xBufferListItem ) );
     }
-    taskEXIT_CRITICAL();
+    ( void ) xTaskResumeAll();

Test Steps

I tested the changes by using ncat: I started multiple instances and had it exchange UDP packets with the DUT. I added stress by running iperf3 at the same time.

Checklist:

• I have tested my changes. No regression in existing tests.
• I have modified and/or added unit-tests to cover the code changes in this Pull Request.

Related Issue

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