rlc_am_lte: replace do_status boolean with atomic

the do_status is queried from the Tx code frequently. To reduce
chances to delay the execution because the RLC Rx side is currently
holding the mutex we can use an atomic.

lib/include/srsran/rlc/rlc_am_lte.h

@@ -527,8 +527,8 @@ private:
     rlc_ringbuffer_t<rlc_amd_rx_pdu>              rx_window;
     std::map<uint32_t, rlc_amd_rx_pdu_segments_t> rx_segments;
 
-    bool poll_received = false;
-    bool do_status     = false;
+    bool              poll_received = false;
+    std::atomic<bool> do_status     = {false}; // light-weight access from Tx entity
 
     /****************************************************************************
      * Timers

lib/src/rlc/rlc_am_lte.cc

@@ -1852,8 +1852,7 @@ void rlc_am_lte::rlc_am_lte_rx::reset_status()
 
 bool rlc_am_lte::rlc_am_lte_rx::get_do_status()
 {
-  std::lock_guard<std::mutex> lock(mutex);
-  return do_status;
+  return do_status.load(std::memory_order_relaxed);
 }
 
 void rlc_am_lte::rlc_am_lte_rx::write_pdu(uint8_t* payload, const uint32_t nof_bytes)