Btrfs: hunting slab corruption

Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
index 1eb333e..b7f3fcb 100644
--- a/fs/btrfs/ctree.c
+++ b/fs/btrfs/ctree.c
@@ -457,6 +457,8 @@
 	int ret;
 	int level;
 
+	WARN_ON(p->nodes[0] != NULL);
+	WARN_ON(!mutex_is_locked(&root->fs_info->fs_mutex));
 again:
 	b = root->node;
 	get_bh(b);