[2.4] [2.5] Fix PPPoATM crash on disconnection (tasklet_disable; kfree(tasklet))

PPPoATM uses tasklet_disable() on a tasklet inside a struct and then frees the struct, leaving a pointer to the freed tasklet inside tasklet lists.
This patch replaces tasklet_disable() with tasklet_kill().

This bug is present in both 2.4.18 and 2.5.15 (and the patch applies to both).

@@ -125,7 +125,7 @@
 	pvcc = atmvcc_to_pvcc(atmvcc);
 	atmvcc->push = pvcc->old_push;
 	atmvcc->pop = pvcc->old_pop;
-	tasklet_disable(&pvcc->wakeup_tasklet);
+	tasklet_kill(&pvcc->wakeup_tasklet);
 	ppp_unregister_channel(&pvcc->chan);
 	atmvcc->user_back = NULL;
 	kfree(pvcc);