output: Add linktype name

Issue: 6954

This commit adds the linktype name to the output stream. The name is
determined from the pcap utility function pcap_datalink_val_to_name

etc/schema.json

@@ -3472,6 +3472,10 @@
             "properties": {
                 "linktype": {
                     "type": "integer"
+                },
+                "linktype_name": {
+                    "type": "string",
+                    "description": "the descriptive name of the linktype"
                 }
             },
             "additionalProperties": false

src/output-json.c

@@ -428,8 +428,15 @@ void EvePacket(const Packet *p, JsonBuilder *js, uint32_t max_length)
         return;
     }
     if (!jb_set_uint(js, "linktype", p->datalink)) {
+        jb_close(js);
         return;
     }
+
+    const char *dl_name = DatalinkValueToName(p->datalink);
+    // Intentionally ignore the return value from jb_set_string and proceed
+    // so the jb object is closed
+    jb_set_string(js, "linktype_name", dl_name == NULL ? "n/a" : dl_name);
+
     jb_close(js);
 }