erofs-utils: mkfs: introduce inner-file multi-threaded compression

erofs-utils: mkfs: introduce inner-file multi-threaded compression

Currently, the creation of EROFS compressed image creation is
single-threaded, which suffers from performance issues. This patch
attempts to address it by compressing the large file in parallel.

Specifically, each input file larger than 16MiB is splited into
segments, and each worker thread compresses a segment as if it were
a separate file.  Finally, the main thread merges all the compressed
segments.

Multi-threaded compression is not compatible with -Ededupe,
-E(all-)fragments for now.

Signed-off-by: Yifan Zhao <zhaoyifan@sjtu.edu.cn>
Co-authored-by: Tong Xin <xin_tong@sjtu.edu.cn>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20240315011019.610442-5-hsiangkao@linux.alibaba.com
Link: https://lore.kernel.org/r/ZfaW3oLe8Q2621DV@debian