btrfs: update outdated comment in __add_block_group_free_space()

btrfs: update outdated comment in __add_block_group_free_space()

The function add_block_group_free_space() was renamed
btrfs_add_block_group_free_space() by commit 6fc5ef782988 ("btrfs:
add btrfs prefix to free space tree exported functions").  Update
the comment accordingly.

Do some reorganization of the next few lines to keep the comment
within 80 characters.

Signed-off-by: Julia Lawall <Julia.Lawall@inria.fr>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>