Plane: Use AIRSPEED_STALL for roll stall prevent.

Similarly to how it's done in the TECS stall prevention, used
AIRSPEED_STALL (if available) in the stall prevention calculations.
Otherwise use a safe approximation based on AIRSPEED_MIN.

ArduPlane/Attitude.cpp

@@ -657,7 +657,13 @@ void Plane::update_load_factor(void)
     }
 #endif
 
-    float max_load_factor = sq(smoothed_airspeed / MAX(aparm.airspeed_min, 1));
+    _TASstall = is_positive(aparm.airspeed_stall)
+                ? aparm.airspeed_stall
+                // Per the param. description, AIRSPEED_MIN should be 20% higher
+                // than the stall speed. Use 15% less to be on the safe side.
+                : aparm.airspeed_min * 0.85;
+
+    float max_load_factor = sq(smoothed_airspeed / MAX(_TASstall, 1));
     if (max_load_factor <= 1) {
         // our airspeed is below the minimum airspeed. Limit roll to
         // 25 degrees