btrfs: abstract out loading the tree root

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>