9p: avoid putting oldfid in p9_client_walk() error path

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{
                    "product": "Linux",
                    "vendor": "Linux",
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                    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
                    "programFiles": [
                        "net/9p/client.c"
                    ],
                    "versions": [
                        {
                            "version": "6.0",
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                        {
                            "version": "6.1.177",
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                            "version": "7.2",
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