erofs-utils: mkfs: implement multi-threaded fragments

erofs-utils: mkfs: implement multi-threaded fragments

Currently, only `-Eall-fragments` is allowed for multi-threaded
compression.  However, in many cases, we don't want the entire file
merged into the packed inode, as it may impact runtime performance.

Let's implement multi-threaded compression for `-Efragments` now,
although it's still not very fast and need to optimize performance
even further for this option.

Note that the image sizes could be larger without `-Ededupe` compared
to `-Eall-fragments` since the head parts aren't deduplicated for now.

Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20250323043451.2907228-1-hsiangkao@linux.alibaba.com