Btrfs: use helper to get used bytes of space_info

This uses a helper instead of open code around used byte of space_info
everywhere.

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
1 file changed