Quoting from upstream patch: On setting up the callback to the client, we attempt to use the same authentication flavor the client did. We find an rpc cred to use by calling rpcauth_lookup_credcache(), which assumes that the given authentication flavor has a credentials cache. However, this is not required to be true--in particular, auth_null does not use one. Instead, we should call the auth's lookup_cred() method. Without this, a client attempting to mount using nfsv4 and auth_null triggers a null dereference. The code was introduced in upstream commit 3cef9ab2 (v2.6.31-rc1), fixed in 886e3b7f (v2.6.32-rc1), and was later replaced by 80fc015b.
Commit 886e3b7f is also in 2.6.31.2 (nfsd4-fix-null-dereference-creating-nfsv4-callback-client.patch), so no fix needed for Fedora 12.