erofs-utils: introduce fragment cache

erofs-utils: introduce fragment cache

Difference from the previous Yiyan's version [1], it just uses
a tmpfile to keep all decompressed data for fragments.

Dataset: linux 5.4.140
mkfs.erofs command line:
	mkfs.erofs -zlzma -C131072 -T0 -Eall-fragments,fragdedupe=inode foo.erofs <dir>
Test command line:
	hyperfine -p "echo 3 > /proc/sys/vm/drop_caches; sleep 1" "fsck/fsck.erofs --extract foo.erofs"

Vanilla:
  Time (mean ± σ):     362.309 s ±  0.406 s   [User: 360.298 s, System: 0.956 s]

After:
  Time (mean ± σ):     20.880 s ±  0.026 s    [User: 19.751 s, System: 1.058 s]

An in-memory LRU cache could also be implemented later to meet
different needs.

[1] https://lore.kernel.org/r/20231023071528.1912105-1-lyy0627@sjtu.edu.cn

Cc: Li Yiyan <lyy0627@sjtu.edu.cn>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20250120023538.3156606-1-hsiangkao@linux.alibaba.com