packetbuf: add packetbuf_remaininglen()

packetbuf_freelen() returns the length of free space in packetbuf.

core/net/packetbuf.c

@@ -184,6 +184,12 @@ packetbuf_totlen(void)
   return packetbuf_hdrlen() + packetbuf_datalen();
 }
 /*---------------------------------------------------------------------------*/
+uint16_t
+packetbuf_remaininglen(void)
+{
+  return PACKETBUF_SIZE - packetbuf_totlen();
+}
+/*---------------------------------------------------------------------------*/
 void
 packetbuf_attr_clear(void)
 {

core/net/packetbuf.h

@@ -123,6 +123,13 @@ uint16_t packetbuf_datalen(void);
  */
 uint16_t packetbuf_totlen(void);
 
+/**
+ * \brief      Get the total length of the remaining space in the packetbuf
+ * \return     Length of the remaining space in the packetbuf
+ *
+ */
+uint16_t packetbuf_remaininglen(void);
+
 /**
  * \brief      Set the length of the data in the packetbuf
  * \param len  The length of the data