Fix available memory calculation.

Also account for buffers, making the value match the output of free(1)

consoles/node.html

@@ -22,7 +22,7 @@
   <td><a href="node-overview.html?instance={{ .Labels.instance }}">{{ reReplaceAll "(.*?://)([^:/]+?)(:\\d+)?/.*" "$2" .Labels.instance }}</a></td>
   <td{{ if eq (. | value) 1.0 }}>Yes{{ else }} class="alert-danger">No{{ end }}</td>
   <td>{{ template "prom_query_drilldown" (args (printf "100 * (1 - avg by(instance)(rate(node_cpu{job='node',mode='idle',instance='%s'}[5m])))" .Labels.instance) "%" "printf.3g") }}</td>
-  <td>{{ template "prom_query_drilldown" (args (printf "node_memory_MemTotal{job='node',instance='%s'} - node_memory_MemFree{job='node',instance='%s'} - node_memory_Cached{job='node',instance='%s'}" .Labels.instance .Labels.instance .Labels.instance) "B" "humanize1024") }}</td>
+  <td>{{ template "prom_query_drilldown" (args (printf "node_memory_MemFree{job='node',instance='%s'} + node_memory_Cached{job='node',instance='%s'} + node_memory_Buffers{job='node',instance='%s'}" .Labels.instance .Labels.instance .Labels.instance) "B" "humanize1024") }}</td>
 </tr>
 {{ else }}
 <tr><td colspan=4>No nodes found.</td></tr>