genirq: Move prio assignment into the newly created thread

With enabled threaded interrupts the nouveau driver reported the
following:
| Chain exists of:
|   &mm->mmap_lock#2 --> &device->mutex --> &cpuset_rwsem
|
|  Possible unsafe locking scenario:
|
|        CPU0                    CPU1
|        ----                    ----
|   lock(&cpuset_rwsem);
|                                lock(&device->mutex);
|                                lock(&cpuset_rwsem);
|   lock(&mm->mmap_lock#2);

The device->mutex is nvkm_device::mutex.

Unblocking the lockchain at `cpuset_rwsem' is probably the easiest thing
to do.
Move the priority assignment to the start of the newly created thread.

Fixes: 710da3c ("sched/core: Prevent race condition between cpuset and __sched_setscheduler()")
Reported-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
[bigeasy: Patch description]
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lkml.kernel.org/r/a23a826af7c108ea5651e73b8fbae5e653f16e86.camel@gmx.de

kernel/irq/manage.c

@@ -1155,6 +1155,8 @@ static int irq_thread(void *data)
 	irqreturn_t (*handler_fn)(struct irq_desc *desc,
 			struct irqaction *action);
 
+	sched_set_fifo(current);
+
 	if (force_irqthreads && test_bit(IRQTF_FORCED_THREAD,
 					&action->thread_flags))
 		handler_fn = irq_forced_thread_fn;
@@ -1320,8 +1322,6 @@ setup_irq_thread(struct irqaction *new, unsigned int irq, bool secondary)
 	if (IS_ERR(t))
 		return PTR_ERR(t);
 
-	sched_set_fifo(t);
-
 	/*
 	 * We keep the reference to the task struct even if
 	 * the thread dies to avoid that the interrupt code