SRSENB: fix MAC UE DB Read protection

srsenb/src/stack/mac/mac.cc

@@ -679,9 +679,12 @@ int mac::get_dl_sched(uint32_t tti_tx_dl, dl_sched_list_t& dl_sched_res_list)
   }
 
   // Count number of TTIs for all active users
+  {
+    srslte::rwlock_read_guard lock(rwlock);
     for (auto& u : ue_db) {
       u.second->metrics_cnt();
     }
+  }
 
   return SRSLTE_SUCCESS;
 }