[ip6] revert allowing sending packet with hop limit 0 (#3092)

src/core/net/ip6.cpp

@@ -138,7 +138,7 @@ otError Ip6::AddTunneledMplOption(Message &aMessage, Header &aHeader, MessageInf
     messageInfo.GetPeerAddr().mFields.m16[7] = HostSwap16(0x00fc);
 
     tunnelHeader.Init();
-    tunnelHeader.SetHopLimit(kDefaultHopLimit);
+    tunnelHeader.SetHopLimit(static_cast<uint8_t>(kDefaultHopLimit));
     tunnelHeader.SetPayloadLength(aHeader.GetPayloadLength() + sizeof(tunnelHeader));
     tunnelHeader.SetDestination(messageInfo.GetPeerAddr());
     tunnelHeader.SetNextHeader(kProtoIp6);
@@ -361,7 +361,7 @@ otError Ip6::SendDatagram(Message &aMessage, MessageInfo &aMessageInfo, IpProto
     header.Init();
     header.SetPayloadLength(payloadLength);
     header.SetNextHeader(aIpProto);
-    header.SetHopLimit(aMessageInfo.mHopLimit);
+    header.SetHopLimit(aMessageInfo.mHopLimit ? aMessageInfo.mHopLimit : static_cast<uint8_t>(kDefaultHopLimit));
 
     if (aMessageInfo.GetSockAddr().IsUnspecified() || aMessageInfo.GetSockAddr().IsMulticast())
     {

src/core/net/socket.hpp

@@ -59,11 +59,7 @@ public:
      * This constructor initializes the object.
      *
      */
-    MessageInfo(void)
-    {
-        memset(this, 0, sizeof(*this));
-        mHopLimit = kDefaultHopLimit;
-    }
+    MessageInfo(void) { memset(this, 0, sizeof(*this)); }
 
     /**
      * This method returns a reference to the local socket address.
@@ -184,12 +180,6 @@ public:
      *
      */
     void SetLinkInfo(const void *aLinkInfo) { mLinkInfo = aLinkInfo; }
-
-private:
-    enum
-    {
-        kDefaultHopLimit = OPENTHREAD_CONFIG_IPV6_DEFAULT_HOP_LIMIT,
-    };
 };
 
 /**