nfsd: RCU-protect cl_cb_session to fix use-after-free on session teardown

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                    "vendor": "Linux",
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                    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
                    "programFiles": [
                        "fs/nfsd/nfs4callback.c",
                        "fs/nfsd/nfs4state.c",
                        "fs/nfsd/state.h",
                        "fs/nfsd/trace.h"
                    ],
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