btrfs-progs: introduce alloc_chunk_ctl structure
    
    Introduce an alloc_chunk_ctl structure, which holds parameters to control
    chunk allocation.
    
    Furthermore use this throughout btrfs_alloc_chunk().
    
    Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
    Signed-off-by: David Sterba <dsterba@suse.com>