fsck.erofs: support --extract=X like fsck.cramfs to extract to path X

[Igor Eisberg]

* Extracts dirs, regular files and symlinks (overwrite enabled with a
erofs_warn, mainly for use with WSL, in case certain files in img exist in
different letter cases under the same dir, i.e. "path/to/file/alarm" and
"path/to/file/Alarm").
* Raw and compressed data chunks are handled with a unified function to
avoid repeats, compressed data is verified lineary
(with EROFS_GET_BLOCKS_FIEMAP) instead of lookback, as it's problematic to
extract data when looping backwards.
* Also refactored some function names and strings for consistency.

Please let me know if anything has to be changed.

Best regards to Mr. Xiang!
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