relayfs: fix infinite loop with splice()

Running kmemtraced, which uses splice() on relayfs, causes a hard lock on
x86-64 SMP.  As described by Tom Zanussi:

  It looks like you hit the same problem as described here:

  commit 8191ecd1

      splice: fix infinite loop in generic_file_splice_read()

  relay uses the same loop but it never got noticed or fixed.

Cc: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Tested-by: Pekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: Tom Zanussi <tzanussi@gmail.com>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/relay.c

@@ -1317,13 +1317,10 @@ static ssize_t relay_file_splice_read(struct file *in,
 		if (ret < 0)
 			break;
 		else if (!ret) {
-			if (spliced)
-				break;
-			if (flags & SPLICE_F_NONBLOCK) {
+			if (flags & SPLICE_F_NONBLOCK)
 				ret = -EAGAIN;
 			break;
 		}
-		}
 
 		*ppos += ret;
 		if (ret > len)