btrfs: tests, use BTRFS_FS_STATE_DUMMY_FS_INFO instead of dummy root

Now that we have a dummy fs_info associated with each test that
uses a root, we don't need the DUMMY_ROOT bit anymore.  This lets
us make choices without needing an actual root like in e.g.
btrfs_find_create_tree_block.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
index 0abed1a..e926b9f 100644
--- a/fs/btrfs/ctree.c
+++ b/fs/btrfs/ctree.c
@@ -1505,7 +1505,7 @@
 				   struct btrfs_root *root,
 				   struct extent_buffer *buf)
 {
-	if (btrfs_test_is_dummy_root(root))
+	if (btrfs_is_testing(root->fs_info))
 		return 0;
 
 	/* ensure we can see the force_cow */