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/backref.c b/fs/btrfs/backref.c
index 1ac8a3c..2b88439 100644
--- a/fs/btrfs/backref.c
+++ b/fs/btrfs/backref.c
@@ -361,7 +361,7 @@
 		goto out;
 	}
 
-	if (btrfs_test_is_dummy_root(root)) {
+	if (btrfs_is_testing(fs_info)) {
 		srcu_read_unlock(&fs_info->subvol_srcu, index);
 		ret = -ENOENT;
 		goto out;