loopback filesystem, growable loopback device

ref:

• https://serverfault.com/questions/166748/performance-of-loopback-filesystems
• https://unix.stackexchange.com/questions/104790/how-to-setup-a-growable-loopback-device
• https://debian-administration.org/article/664/Creating_dynamic_volumes_with_loop_devices

就是做出一個loopback filesystem，是用多少，長多少，不用先 dd 一個位子出來。