[I/OAT]: Remove CPU hotplug lock from net_dma_rebalance

Remove the lock_cpu_hotplug()/unlock_cpu_hotplug() calls from
net_dma_rebalance

The lock_cpu_hotplug()/unlock_cpu_hotplug() sequence in
net_dma_rebalance is both incorrect (as pointed out by David Miller)
because lock_cpu_hotplug() may sleep while the net_dma_event_lock
spinlock is held, and unnecessary (as pointed out by Andrew Morton) as
spin_lock() disables preemption which protects from CPU hotplug
events.

Signed-off-by: Chris Leech <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/core/dev.c

···
 	unsigned int cpu, i, n;
 	struct dma_chan *chan;
 
-	lock_cpu_hotplug();
-
 	if (net_dma_count == 0) {
 		for_each_online_cpu(cpu)
 			rcu_assign_pointer(per_cpu(softnet_data.net_dma, cpu), NULL);
-		unlock_cpu_hotplug();
 		return;
 	}
 
···
 		i++;
 	}
 	rcu_read_unlock();
-
-	unlock_cpu_hotplug();
 }
 
 /**