mptcp: allow subflow rcv wnd to shrink

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{
                    "product": "Linux",
                    "vendor": "Linux",
                    "defaultStatus": "affected",
                    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
                    "programFiles": [
                        "net/mptcp/options.c"
                    ],
                    "versions": [
                        {
                            "version": "5.19",
                            "status": "affected"
                        },
                        {
                            "version": "0",
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                            "versionType": "semver"
                        },
                        {
                            "version": "6.1.176",
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                        },
                        {
                            "version": "6.6.143",
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                        },
                        {
                            "version": "6.12.94",
                            "lessThanOrEqual": "6.12.*",
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                        },
                        {
                            "version": "6.18.36",
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                            "version": "7.0.13",
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                        {
                            "version": "7.1",
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                            "versionType": "original_commit_for_fix"
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                }