block: prevent division by zero in blk_rq_stat_sum()

{
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                    "vendor": "Linux",
                    "defaultStatus": "unaffected",
                    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
                    "programFiles": [
                        "block/blk-stat.c"
                    ],
                    "versions": [
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                }

{
                    "product": "Linux",
                    "vendor": "Linux",
                    "defaultStatus": "affected",
                    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
                    "programFiles": [
                        "block/blk-stat.c"
                    ],
                    "versions": [
                        {
                            "version": "4.15",
                            "status": "affected"
                        },
                        {
                            "version": "0",
                            "lessThan": "4.15",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "4.19.312",
                            "lessThanOrEqual": "4.19.*",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "5.4.274",
                            "lessThanOrEqual": "5.4.*",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "5.10.215",
                            "lessThanOrEqual": "5.10.*",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "5.15.155",
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                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "6.1.86",
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                        },
                        {
                            "version": "6.6.27",
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                        },
                        {
                            "version": "6.8.6",
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                            "versionType": "semver"
                        },
                        {
                            "version": "6.9",
                            "lessThanOrEqual": "*",
                            "status": "unaffected",
                            "versionType": "original_commit_for_fix"
                        }
                    ]
                }