DAOS-13938 dfuse: adjust offset in readdir cache entry list

[jolivier23]

The dfuse readdir caching feature stats entries returned by dfs readdir that might be legimitely deleted by another process or another node. When this happens, the dfuse code decides to skip the deleted entry, but might leave the readdir entry cache in an inconsistent state causing the following assertion failure in dfuse_dre_drop():
drc->drc_next_offset == expected_offset + 1 || drc->drc_next_offset == READDIR_EOD

This patch fixes the offset in the linked list of cached readdir entries to support skipped entries.

Change-Id: Icbd5abb648110559851002301e758128e4ced2c7

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[github-actions]

Ticket title is 'dFuse process crashes with the following error sporadically on 2.4 branch'
Status is 'Reopened'
Labels: 'GCP,google-cloud-daos'
https://daosio.atlassian.net/browse/DAOS-13938