os: allow direntries to have zero inodes on Linux

os: allow direntries to have zero inodes on Linux

Some Linux filesystems have been known to return valid enties with
zero inodes. This new behavior also puts Go in agreement with recent
glibc.

Fixes #76428

Change-Id: Ieaf50739a294915a3ea2ef8c5a3bb2a91a186881
GitHub-Last-Rev: 8f83d009ef0320fd3fe7cf03e55d5d24df57f015
GitHub-Pull-Request: golang/go#76448
Reviewed-on: https://go-review.googlesource.com/c/go/+/724220
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>