[stats] allow knet_handle_get_stats to operate in a readlock context

• add global stat mutex lock to protect stats updates
• use global stat mutex lock across all the threads
• fix up some minor bugs:
  • update RX crypto stats only when crypto is enabled
  • update compress and crypto stats in a consistent fashion

Signed-off-by: Fabio M. Di Nitto <fdinitto@redhat.com>