netfs: Fix missing locking around retry adding new subreqs

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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",
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                        "fs/netfs/read_retry.c",
                        "fs/netfs/write_retry.c"
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