Rover: follow mode restores offsets to zero on exit

APMrover2/mode.h

@@ -630,6 +630,7 @@ public:
 protected:
 
     bool _enter() override;
+    void _exit() override;
 };
 
 class ModeSimple : public Mode

APMrover2/mode_follow.cpp

@@ -14,6 +14,12 @@ bool ModeFollow::_enter()
     return true;
 }
 
+// exit handling
+void ModeFollow::_exit()
+{
+    g2.follow.clear_offsets_if_required();
+}
+
 void ModeFollow::update()
 {
     // stop vehicle if no speed estimate