[meshcop] add MeshLocalPrefixTlv::GetMeshLocalPrefixLength() (#3930)

src/core/meshcop/dataset_manager_ftd.cpp

@@ -150,7 +150,7 @@ otError DatasetManager::HandleSet(Coap::Message &aMessage, const Ip6::MessageInf
     if (Tlv::GetTlv(aMessage, Tlv::kMeshLocalPrefix, sizeof(meshLocalPrefix), meshLocalPrefix) == OT_ERROR_NONE &&
         meshLocalPrefix.IsValid() &&
         memcmp(&meshLocalPrefix.GetMeshLocalPrefix(), &Get<Mle::MleRouter>().GetMeshLocalPrefix(),
-               meshLocalPrefix.GetLength()))
+               meshLocalPrefix.GetMeshLocalPrefixLength()))
     {
         doesAffectConnectivity = true;
     }

src/core/meshcop/meshcop_tlvs.hpp

@@ -606,6 +606,14 @@ public:
      */
     bool IsValid(void) const { return GetLength() >= sizeof(*this) - sizeof(Tlv); }
 
+    /**
+     * This method returns the size (in bytes) of the Mesh Local Prefix field.
+     *
+     * @returns The size (in bytes) of the Mesh Local Prefix field (8 bytes).
+     *
+     */
+    uint8_t GetMeshLocalPrefixLength(void) const { return sizeof(mMeshLocalPrefix); }
+
     /**
      * This method returns the Mesh Local Prefix value.
      *