reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/netfilter/nf-next.git/commit/?id=64d7d5abe2160bba369b4a8f06bdf5630573bab0
        git remote add netfilter-nf-next https://git.kernel.org/pub/scm/linux/kernel/git/netfilter/nf-next.git
        git fetch --no-tags netfilter-nf-next for-netdev-nf-next-26-06-14
        git checkout 64d7d5abe2160bba369b4a8f06bdf5630573bab0
        # apt-get install sparse
        # sparse: v0.6.5-rc1

        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 C=1 CF='-fdiagnostic-prefix -D__CHECK_ENDIAN__ -fmax-errors=unlimited -fmax-warnings=unlimited' O=build_dir ARCH=x86_64 olddefconfig
        make W=1 C=1 CF='-fdiagnostic-prefix -D__CHECK_ENDIAN__ -fmax-errors=unlimited -fmax-warnings=unlimited' O=build_dir ARCH=x86_64 SHELL=/bin/bash net/netfilter/