powerpc/eeh: Use interruptible sleep in keehd

To replace down() with down_interrutible() to avoid following
warning:

[c00000007ba7b710] [c000000000014410] .__switch_to+0x1b0/0x380
[c00000007ba7b7c0] [c0000000007b408c] .__schedule+0x3ec/0x970
[c00000007ba7ba50] [c0000000007b1f24] .schedule_timeout+0x1a4/0x2b0
[c00000007ba7bb30] [c0000000007b34a4] .__down+0xa4/0x104
[c00000007ba7bbf0] [c0000000000b9230] .down+0x60/0x70
[c00000007ba7bc80] [c0000000000336d0] .eeh_event_handler+0x70/0x190
[c00000007ba7bd30] [c0000000000b1a58] .kthread+0xe8/0xf0
[c00000007ba7be30] [c00000000000a05c] .ret_from_kernel_thread+0x5c/0x8

This also avoids keeping the load average up while doing nothing.
Signed-off-by: Gavin Shan <shangw@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/eeh_event.c

@@ -55,7 +55,8 @@ static int eeh_event_handler(void * dummy)
 	struct eeh_pe *pe;
 
 	while (!kthread_should_stop()) {
-		down(&eeh_eventlist_sem);
+		if (down_interruptible(&eeh_eventlist_sem))
+			break;
 
 		/* Fetch EEH event from the queue */
 		spin_lock_irqsave(&eeh_eventlist_lock, flags);