We consistently use ->num_bytes everywhere through the delayed ref code,
except in btrfs_ref.  Rename btrfs_ref to match all the other code.
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>