gnrc_sixlowpan_frag_fb: introduce function to get entry by tag

sys/include/net/gnrc/sixlowpan/frag/fb.h

@@ -70,6 +70,16 @@ void gnrc_sixlowpan_frag_fb_reset(void);
  */
 gnrc_sixlowpan_frag_fb_t *gnrc_sixlowpan_frag_fb_get(void);
 
+/**
+ * @brief   Get an existing fragmentation buffer entry by a given tag
+ *
+ * @param[in] tag   Tag to search for.
+ *
+ * @return a fragmentation buffer entry if one with @p tag exists.
+ * @return NULL, if no fragmentation buffer entry can be found.
+ */
+gnrc_sixlowpan_frag_fb_t *gnrc_sixlowpan_frag_fb_get_by_tag(uint16_t tag);
+
 /**
  * @brief   Generate a new datagram tag for sending
  *

sys/net/gnrc/network_layer/sixlowpan/frag/fb/gnrc_sixlowpan_frag_fb.c

@@ -46,6 +46,16 @@ gnrc_sixlowpan_frag_fb_t *gnrc_sixlowpan_frag_fb_get(void)
     return NULL;
 }
 
+gnrc_sixlowpan_frag_fb_t *gnrc_sixlowpan_frag_fb_get_by_tag(uint16_t tag)
+{
+    for (unsigned i = 0; i < GNRC_SIXLOWPAN_FRAG_FB_SIZE; i++) {
+        if ((_fbs[i].pkt != NULL) && (_fbs[i].tag == tag)) {
+            return &_fbs[i];
+        }
+    }
+    return NULL;
+}
+
 uint16_t gnrc_sixlowpan_frag_fb_next_tag(void)
 {
     return (++_current_tag);