We're going to be adding more roots that need to be loaded from the
super block, so abstract out the code to read the tree_root from the
super block, and use this helper for the chunk root as well.  This will
make it simpler to load the new trees in the future.
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>