direct-io: Use return from cmpxchg to decide of assignment happened

Not using the return value can in the generic case be racy, so it's
in general good practice to check the return value instead.

This also resolved the warning caused on ARM and other architectures:

fs/direct-io.c: In function 'sb_init_dio_done_wq':
fs/direct-io.c:557:2: warning: value computed is not used [-Wunused-value]

Signed-off-by: Olof Johansson <[e-mail]>
Reviewed-by: Jan Kara <[e-mail]>
Cc: Geert Uytterhoeven <[e-mail]>
Cc: Stephen Rothwell <[e-mail]>
Cc: Al Viro <[e-mail]>
Cc: Christoph Hellwig <[e-mail]>
Cc: Russell King <[e-mail]>
Cc: H Peter Anvin <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/direct-io.c

···
  */
 static int sb_init_dio_done_wq(struct super_block *sb)
 {
+	struct workqueue_struct *old;
 	struct workqueue_struct *wq = alloc_workqueue("dio/%s",
 						      WQ_MEM_RECLAIM, 0,
 						      sb->s_id);
···
 	/*
 	 * This has to be atomic as more DIOs can race to create the workqueue
 	 */
-	cmpxchg(&sb->s_dio_done_wq, NULL, wq);
+	old = cmpxchg(&sb->s_dio_done_wq, NULL, wq);
 	/* Someone created workqueue before us? Free ours... */
-	if (wq != sb->s_dio_done_wq)
+	if (old)
 		destroy_workqueue(wq);
 	return 0;
 }