smb/server: fix invalid pointer dereference in ksmbd_stop_durable_scavenger()

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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": [
                        "fs/smb/server/vfs_cache.c"
                    ],
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