erspan: make sure erspan_base_hdr is present in skb->head

{
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                    "vendor": "Linux",
                    "defaultStatus": "unaffected",
                    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
                    "programFiles": [
                        "net/ipv4/ip_gre.c",
                        "net/ipv6/ip6_gre.c"
                    ],
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                        },
                        {
                            "version": "4.20.7",
                            "lessThan": "4.21",
                            "status": "affected",
                            "versionType": "semver"
                        }
                    ]
                }

{
                    "product": "Linux",
                    "vendor": "Linux",
                    "defaultStatus": "affected",
                    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
                    "programFiles": [
                        "net/ipv4/ip_gre.c",
                        "net/ipv6/ip6_gre.c"
                    ],
                    "versions": [
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                            "version": "5.0",
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                        {
                            "version": "0",
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                        },
                        {
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                        },
                        {
                            "version": "5.10.215",
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                        },
                        {
                            "version": "5.15.154",
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                        },
                        {
                            "version": "6.1.85",
                            "lessThanOrEqual": "6.1.*",
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                        },
                        {
                            "version": "6.6.26",
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                        },
                        {
                            "version": "6.8.5",
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                        },
                        {
                            "version": "6.9",
                            "lessThanOrEqual": "*",
                            "status": "unaffected",
                            "versionType": "original_commit_for_fix"
                        }
                    ]
                }