Change worker validation to check USS not PSS

Why? USS is a more reliable mechanism for tracking workers with runaway
memory growth. PSS is great, until the server process that forks new
processes grows large. As each new worker is forked, it inherits a share
of the large amount of the parent process' memory and therefore starts
with a large PSS, possibly exceeding our limits before doing any work.
USS only measures a process' private memory and is a better indicator
when a process is responsible for allocating too much memory without
freeing it.

app/models/miq_server/worker_management/monitor/validation.rb

@@ -20,8 +20,8 @@ def validate_worker(w)
 
     return true unless worker_get_monitor_status(w.pid).nil?
 
-    # Proportional set size is only implemented on linux
-    usage = w.proportional_set_size || w.memory_usage
+    # Unique set size is only implemented on linux
+    usage = w.unique_set_size || w.memory_usage
     if MiqWorker::STATUSES_CURRENT.include?(w.status) && usage_exceeds_threshold?(usage, memory_threshold)
       msg = "#{w.format_full_log_msg} process memory usage [#{usage}] exceeded limit [#{memory_threshold}], requesting worker to exit"
       _log.warn(msg)