net.c: coap_option_check_critical() not updating 'unknown' by ref

'unknown' is updated by coap_option_check_critical if there is a critical
option, but as it is not passed in by reference, any caller does not see
any change to 'unknown'.

Pass in 'unknown' by reference.

Issue got missed in change e2e7830

include/coap3/coap_net_internal.h

@@ -309,7 +309,7 @@ void coap_dispatch(coap_context_t *context, coap_session_t *session,
  */
 int coap_option_check_critical(coap_context_t *ctx,
                                coap_pdu_t *pdu,
-                               coap_opt_filter_t unknown);
+                               coap_opt_filter_t *unknown);
 
 /**
  * Creates a new response for given @p request with the contents of @c