#91 Fix: in case of filesystem corruption, prevent lookup soft lockup

This is a new fix that continues from pull request #88

In this code, it ensures in case of nr_blk == 0, the system does not soft lockup, and return -EUCLEAN, because nr_blk == 0 means the filesystem is corrupted.

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Summary by cubic

Prevent lookup soft lockups on corrupted directory entries. In __file_lookup, return -EUCLEAN when dblock->files[_fi].nr_blk == 0, and mark the check unlikely() as an edge case.