Core/Movement: Use correct root check

Port From (https://github.com/TrinityCore/TrinityCore/commit/dc4b516844e4457b8efce20a521ea3d36dcc42d6)

Source/Game/Movement/MotionMaster.cs

@@ -860,6 +860,10 @@ namespace Game.Movement
             if (Math.Abs(_owner.GetPositionZ() - tz) < 0.1f)
                 return;
 
+            // rooted units don't move (also setting falling+root flag causes client freezes)
+                if (_owner.HasUnitState(UnitState.Root))
+                    return;
+
             _owner.SetFall(true);
 
             // Don't run spline movement for players