btrfs: unify the btrfs_add_delayed_*_ref helpers into one helper

btrfs: unify the btrfs_add_delayed_*_ref helpers into one helper

Now that these helpers are identical, create a helper function that
handles everything properly and strip the individual helpers down to use
just the common helper. This cleans up a significant amount of
duplicated 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>