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 8191ecd1d14c6914c660dfa007154860a7908857

splice: fix infinite loop in generic_file_splice_read()

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

Cc: Mathieu Desnoyers <[e-mail]>
Tested-by: Pekka Enberg <[e-mail]>
Signed-off-by: Tom Zanussi <[e-mail]>
Signed-off-by: Pekka Enberg <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

kernel/relay.c

···
 		if (ret < 0)
 			break;
 		else if (!ret) {
-			if (spliced)
-				break;
-			if (flags & SPLICE_F_NONBLOCK) {
+			if (flags & SPLICE_F_NONBLOCK)
 				ret = -EAGAIN;
-				break;
-			}
+			break;
 		}
 
 		*ppos += ret;