Merge Pull Request from: @gewus 
 
    Compressed inode may suffer read performance issue due to it can not
    use extent cache, so I propose to add this unaligned extent support
    to improve it.

    Currently, it only works in readonly format f2fs image.

    Unaligned extent: in one compressed cluster, physical block number
    will be less than logical block number, so we add an extra physical
    block length in extent info in order to indicate such extent status.

    The idea is if one whole cluster blocks are contiguous physically,
    once its mapping info was readed at first time, we will cache an
    unaligned (or aligned) extent info entry in extent cache, it expects
    that the mapping info will be hitted when rereading cluster. 
 
Link:https://gitee.com/openeuler/kernel/pulls/172 
Reviewed-by: Zheng Zengkai <zhengzengkai@huawei.com> 
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>