reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs.git/commit/?id=c6553fbd9d8fee3e5d07dc6596e02f7289dbfbf6
        git remote add brauner-vfs https://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs.git
        git fetch --no-tags brauner-vfs work.procfs.cleanup
        git checkout c6553fbd9d8fee3e5d07dc6596e02f7289dbfbf6
        # apt-get install llvm-dev

        # save the config file
        make W=1 ARCH=i386 olddefconfig
        make W=1 ARCH=i386 
        make W=1 headers_install
        make W=1 -C tools/testing/selftests