reproduce:
        # apt-get install sparse
        # sparse version: v0.6.4-66-g0196afe1
        # https://github.com/multipath-tcp/mptcp_net-next/commit/b0f2b972f6eeb989a7118b49bd45f0d52f962218
        git remote add mptcp https://github.com/multipath-tcp/mptcp_net-next.git
        git fetch --no-tags mptcp export-net
        git checkout b0f2b972f6eeb989a7118b49bd45f0d52f962218
        # 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=i386 olddefconfig
        make W=1 C=1 CF='-fdiagnostic-prefix -D__CHECK_ENDIAN__ -fmax-errors=unlimited -fmax-warnings=unlimited' O=build_dir ARCH=i386 SHELL=/bin/bash