s390/pkey: Use kfree_sensitive() to fix Coccinelle warnings

{
                    "product": "Linux",
                    "vendor": "Linux",
                    "defaultStatus": "unaffected",
                    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
                    "programFiles": [
                        "drivers/s390/crypto/pkey_api.c"
                    ],
                    "versions": [
                        {
                            "version": "e80d4af0a320972aac58e2004d0ba4e44ef4c5c7",
                            "lessThan": "f2e43af834fcc652b324384f14b561cf610742d4",
                            "status": "affected",
                            "versionType": "git"
                        },
                        {
                            "version": "e80d4af0a320972aac58e2004d0ba4e44ef4c5c7",
                            "lessThan": "62151a0acde90823bdfa991d598c85cf4b1d387d",
                            "status": "affected",
                            "versionType": "git"
                        },
                        {
                            "version": "e80d4af0a320972aac58e2004d0ba4e44ef4c5c7",
                            "lessThan": "22e6824622e8a8889df0f8fc4ed5aea0e702a694",
                            "status": "affected",
                            "versionType": "git"
                        }
                    ]
                }

{
                    "product": "Linux",
                    "vendor": "Linux",
                    "defaultStatus": "affected",
                    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
                    "programFiles": [
                        "drivers/s390/crypto/pkey_api.c"
                    ],
                    "versions": [
                        {
                            "version": "4.11",
                            "status": "affected"
                        },
                        {
                            "version": "0",
                            "lessThan": "4.11",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "6.1.191",
                            "lessThanOrEqual": "6.1.*",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "6.9.9",
                            "lessThanOrEqual": "6.9.*",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "6.10",
                            "lessThanOrEqual": "*",
                            "status": "unaffected",
                            "versionType": "original_commit_for_fix"
                        }
                    ]
                }