[mac] add Mac::GenerateRandomPanId() (#4283)

This commit adds a helper function `Mac::GenerateRandomPanId()` which
generates a random PAN Identifier (excluding the broadcast PAN Id
`kPanIdBroadcast`).

src/core/mac/mac_types.cpp

@@ -42,6 +42,18 @@
 namespace ot {
 namespace Mac {
 
+PanId GenerateRandomPanId(void)
+{
+    PanId panId;
+
+    do
+    {
+        panId = Random::NonCrypto::GetUint16();
+    } while (panId == kPanIdBroadcast);
+
+    return panId;
+}
+
 void ExtAddress::GenerateRandom(void)
 {
     Random::NonCrypto::FillBuffer(m8, sizeof(ExtAddress));

src/core/mac/mac_types.hpp

@@ -73,6 +73,14 @@ typedef otPanId PanId;
  */
 typedef otShortAddress ShortAddress;
 
+/**
+ * This function generates a random IEEE 802.15.4 PAN ID.
+ *
+ * @returns A randomly generated IEEE 802.15.4 PAN ID (excluding `kPanIdBroadcast`).
+ *
+ */
+PanId GenerateRandomPanId(void);
+
 /**
  * This structure represents an IEEE 802.15.4 Extended Address.
  *

src/core/meshcop/dataset_manager_ftd.cpp

@@ -345,10 +345,7 @@ otError ActiveDataset::CreateNewNetwork(otOperationalDataset &aDataset)
     aDataset.mChannel     = preferredChannels.ChooseRandomChannel();
     aDataset.mChannelMask = supportedChannels.GetMask();
 
-    do
-    {
-        aDataset.mPanId = Random::NonCrypto::GetUint16();
-    } while (aDataset.mPanId == Mac::kPanIdBroadcast);
+    aDataset.mPanId = Mac::GenerateRandomPanId();
 
     snprintf(aDataset.mNetworkName.m8, sizeof(aDataset.mNetworkName), "OpenThread-%04x", aDataset.mPanId);
 

src/core/thread/mesh_forwarder.cpp

@@ -496,14 +496,7 @@ otError MeshForwarder::HandleFrameRequest(Mac::TxFrame &aFrame)
             // value.
             if (mSendMessage->GetPanId() == Mac::kPanIdBroadcast && Get<Mac::Mac>().GetPanId() == Mac::kPanIdBroadcast)
             {
-                uint16_t panid;
-
-                do
-                {
-                    panid = Random::NonCrypto::GetUint16();
-                } while (panid == Mac::kPanIdBroadcast);
-
-                Get<Mac::Mac>().SetPanId(panid);
+                Get<Mac::Mac>().SetPanId(Mac::GenerateRandomPanId());
             }
         }