erofs-utils: mkfs: implement extent-based deduplication

erofs-utils: mkfs: implement extent-based deduplication

Currently, only rolling-hash deduplication could be used for
compressed data, and it is still single-threaded for now.

Before applying multi-threaded compression to that, let's allow
extent-based compressed data deduplication if `-Efragments` is on.

This feature will only work if multi-threaded compression is active.

Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20250403081403.2671077-3-hsiangkao@linux.alibaba.com