sched,drbd: Convert to sched_set_fifo*()

Because SCHED_FIFO is a broken scheduler model (see previous patches)
take away the priority field, the kernel can't possibly make an
informed decision.

In this case, use fifo_low, because it only cares about being above
SCHED_NORMAL. Effectively changes prio from 2 to 1.

Cc: axboe@kernel.dk
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Ingo Molnar <mingo@kernel.org>

drivers/block/drbd/drbd_receiver.c

@@ -6019,9 +6019,8 @@ int drbd_ack_receiver(struct drbd_thread *thi)
 	unsigned int header_size = drbd_header_size(connection);
 	int expect   = header_size;
 	bool ping_timeout_active = false;
-	struct sched_param param = { .sched_priority = 2 };
 
-	rv = sched_setscheduler(current, SCHED_RR, &param);
+	rv = sched_set_fifo_low(current);
 	if (rv < 0)
 		drbd_err(connection, "drbd_ack_receiver: ERROR set priority, ret=%d\n", rv);