erofs-utils: introduce incremental builds

erofs-utils: introduce incremental builds

This introduces incremental build support for mkfs, where new on-disk
(meta)data will be appended in a log-structured manner, except for the
root inode (due to current on-disk limitations), as illustrated below:

 ___________________________________________
| base | delta 0 | delta 1 | .. | delta n-1 |
|______|_________|_________|____|___________|
                                   ---> image/data growth direction

Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20240618082414.47876-4-hsiangkao@linux.alibaba.com