fix rewinddir on nfs. fix PR/42879 (and probably PR/40229.)

- remove a wrong _DIAGASSERT
- update comments
- whitespace

Fix a bug in the implementation of seekdir(). If the first entry in
a block has been unlinked, seekdir() may overshoot by one entry.
Thus, _readdir_unlinked() must not skip deleted entries when being

Fix a bug in the implementation of seekdir(). If the first entry in
a block has been unlinked, seekdir() may overshoot by one entry.
Thus, _readdir_unlinked() must not skip deleted entries when being
called from seekdir().
Christos agreed.

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PR/24324: Arne H Juul: Re-implement seekdir/telldir using a pointer of
locations per directory instead of a global hash table to avoid memory
leak issues, and incorrect results.