MINOR: task: stop abusing the nice field to detect a tasklet

It's cleaner to use a flag from the task's state to detect a tasklet
and it's even cheaper. One of the best benefits is that this will
allow to get the nice field out of the common part since the tasklet
doesn't need it anymore. This commit uses the last task bit available
but that's temporary as the purpose of the change is to extend this.
diff --git a/src/task.c b/src/task.c
index 4c73b8c..350146d 100644
--- a/src/task.c
+++ b/src/task.c
@@ -487,7 +487,7 @@
 
 		budgets[queue]--;
 		t = (struct task *)LIST_ELEM(tl_queues[queue].n, struct tasklet *, list);
-		state = t->state & (TASK_SHARED_WQ|TASK_SELF_WAKING|TASK_HEAVY|TASK_KILLED);
+		state = t->state & (TASK_SHARED_WQ|TASK_SELF_WAKING|TASK_HEAVY|TASK_F_TASKLET|TASK_KILLED);
 
 		ti->flags &= ~TI_FL_STUCK; // this thread is still running
 		activity[tid].ctxsw++;
@@ -498,7 +498,7 @@
 
 		_HA_ATOMIC_SUB(&sched->rq_total, 1);
 
-		if (TASK_IS_TASKLET(t)) {
+		if (state & TASK_F_TASKLET) {
 			uint64_t before = 0;
 
 			LIST_DEL_INIT(&((struct tasklet *)t)->list);