BUG/MINOR: task: allow to use tasklet_wakeup_after with tid -1

Adjust BUG_ON() statement to allow tasklet_wakeup_after() for tasklets
with tid pinned to -1 (the current thread). This is similar to
tasklet_wakeup().

This should be backported up to 2.6.
diff --git a/src/task.c b/src/task.c
index 990ab42..12b24b4 100644
--- a/src/task.c
+++ b/src/task.c
@@ -170,7 +170,7 @@
  */
 struct list *__tasklet_wakeup_after(struct list *head, struct tasklet *tl)
 {
-	BUG_ON(tid != tl->tid);
+	BUG_ON(tl->tid >= 0 && tid != tl->tid);
 	/* this tasklet runs on the caller thread */
 	if (!head) {
 		if (tl->state & TASK_HEAVY) {