AP_InertialSensor: stop sensors converging if motors arm

if the user arms within 30s of startup then stop the re-init of the
sensors. This can give less accurate frequency as the sample rate may
not have settled yet, but it is better than doing init of the filters
while the vehicle may be flying

also fix a 32 bit millis wrap

libraries/AP_InertialSensor/AP_InertialSensor_Backend.cpp

@@ -54,6 +54,15 @@ void AP_InertialSensor_Backend::_set_gyro_oversampling(uint8_t instance, uint8_t
     _imu._gyro_over_sampling[instance] = n;
 }
 
+/*
+  while sensors are converging to get the true sample rate we re-init the notch filters.
+  stop doing this if the user arms
+ */
+bool AP_InertialSensor_Backend::sensors_converging() const
+{
+    return AP_HAL::millis64() < HAL_INS_CONVERGANCE_MS && !hal.util->get_soft_armed();
+}
+
 /*
   update the sensor rate for FIFO sensors
 

libraries/AP_InertialSensor/AP_InertialSensor_Backend.h

@@ -224,7 +224,7 @@ class AP_InertialSensor_Backend
     void _update_sensor_rate(uint16_t &count, uint32_t &start_us, float &rate_hz) const __RAMFUNC__;
 
     // return true if the sensors are still converging and sampling rates could change significantly
-    bool sensors_converging() const { return AP_HAL::millis() < HAL_INS_CONVERGANCE_MS; }
+    bool sensors_converging() const;
 
     // set accelerometer max absolute offset for calibration
     void _set_accel_max_abs_offset(uint8_t instance, float offset);