fs/btrfs: use WARN

Use WARN rather than printk followed by WARN_ON(1), for conciseness.

A simplified version of the semantic patch that makes this transformation
is as follows: (http://coccinelle.lip6.fr/)

// <smpl>
@@
expression list es;
@@

-printk(
+WARN(1,
  es);
-WARN_ON(1);
// </smpl>

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Reviewed-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>

fs/btrfs/volumes.c

@@ -3323,9 +3323,8 @@ static int __btrfs_alloc_chunk(struct btrfs_trans_handle *trans,
 		cur = cur->next;
 
 		if (!device->writeable) {
-			printk(KERN_ERR
+			WARN(1, KERN_ERR
 			       "btrfs: read-only device in alloc_list\n");
-			WARN_ON(1);
 			continue;
 		}