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/extent-tree.c

@@ -6292,10 +6292,9 @@ use_block_rsv(struct btrfs_trans_handle *trans,
 		static DEFINE_RATELIMIT_STATE(_rs,
 				DEFAULT_RATELIMIT_INTERVAL,
 				/*DEFAULT_RATELIMIT_BURST*/ 2);
-		if (__ratelimit(&_rs)) {
-			printk(KERN_DEBUG "btrfs: block rsv returned %d\n", ret);
-			WARN_ON(1);
-		}
+		if (__ratelimit(&_rs))
+			WARN(1, KERN_DEBUG "btrfs: block rsv returned %d\n",
+			     ret);
 		ret = reserve_metadata_bytes(root, block_rsv, blocksize,
 					     BTRFS_RESERVE_NO_FLUSH);
 		if (!ret) {