gnrc_pktbuf: add gnrc_pktbuf_reverse_snips() helper function

This allows for

a) testing the packet reversal properly in unittests
b) use it in other places than `gnrc_ipv6`'s receive function

sys/include/net/gnrc/pktbuf.h

@@ -225,6 +225,24 @@ gnrc_pktsnip_t *gnrc_pktbuf_remove_snip(gnrc_pktsnip_t *pkt, gnrc_pktsnip_t *sni
  */
 gnrc_pktsnip_t *gnrc_pktbuf_replace_snip(gnrc_pktsnip_t *pkt, gnrc_pktsnip_t *old, gnrc_pktsnip_t *add);
 
+/**
+ * @brief   Reverses snip order of a packet in a write-protected manner.
+ *
+ * This can be used to change the send/receive order of a packet (see
+ * @ref gnrc_pktsnip_t)
+ *
+ * @note    @p pkt is released on failure.
+ *
+ * @param[in] pkt   A packet. When this function fails (due to a full packet
+ *                  packet buffer) @p pkt will be released.
+ *
+ * @return  The reversed version of @p pkt on success
+ * @return  NULL, when there is not enough space in the packet buffer to reverse
+ *          the packet in a write-protected manner. @p pkt is released in that
+ *          case.
+ */
+gnrc_pktsnip_t *gnrc_pktbuf_reverse_snips(gnrc_pktsnip_t *pkt);
+
 /**
  * @brief Duplicates pktsnip chain upto (including) a snip with the given type
  *        as a continuous snip.

sys/net/gnrc/pktbuf/gnrc_pktbuf.c

@@ -86,5 +86,28 @@ gnrc_pktsnip_t *gnrc_pktbuf_replace_snip(gnrc_pktsnip_t *pkt,
     return pkt;
 }
 
+gnrc_pktsnip_t *gnrc_pktbuf_reverse_snips(gnrc_pktsnip_t *pkt)
+{
+    gnrc_pktsnip_t *reversed = NULL, *ptr = pkt;
+
+    while (ptr != NULL) {
+        gnrc_pktsnip_t *next;
+
+        /* try to write-protect snip as its next-pointer is changed below */
+        pkt = gnrc_pktbuf_start_write(ptr); /* use pkt as temporary variable */
+        if (pkt == NULL) {
+            gnrc_pktbuf_release(reversed);
+            gnrc_pktbuf_release(ptr);
+            return NULL;
+        }
+        /* switch around pointers */
+        next = pkt->next;
+        pkt->next = reversed;
+        reversed = pkt;
+        ptr = next;
+    }
+    return reversed;
+}
+
 
 /** @} */