btrfs: move ->parent and ->ref_root into btrfs_delayed_ref_node

btrfs: move ->parent and ->ref_root into btrfs_delayed_ref_node

These two members are shared by both the tree refs and data refs, so
move them into btrfs_delayed_ref_node proper.  This allows us to greatly
simplify the comparison code, as the shared refs always only sort on
parent, and the non shared refs always sort first on ref_root, and then
only data refs sort on their specific fields.

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>