btrfs-progs: introduce init_alloc_chunk_ctl
    
    Factor out setting of alloc_chuk_ctl fileds in a separate function
    init_alloc_chunk_ctl.
    
    Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
    Signed-off-by: David Sterba <dsterba@suse.com>