Rename netlink.py to linux_netlink.py

This is to comply to a comment from @cznewt in
https://github.com/tcpcloud/salt-formula-heka/pull/24.

linux/meta/collectd.yml

@@ -4,7 +4,7 @@ local_plugin:
     template: linux/files/collectd_netlink.conf
     ignore_selected: false
     interfaces:
-    {%- for interface_name in salt['netlink.ls']() %}
+    {%- for interface_name in salt['linux_netlink.ls']() %}
     - {{ interface_name }}
     {%- endfor %}
   linux_system_cpu: