reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/sandeen/linux.git/commit/?id=af0a8e5ab96dba92a0402ca56407a9e5f656403c
        git remote add sandeen https://git.kernel.org/pub/scm/linux/kernel/git/sandeen/linux.git
        git fetch --no-tags sandeen mount-api
        git checkout af0a8e5ab96dba92a0402ca56407a9e5f656403c
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 O=build_dir ARCH=i386 olddefconfig
        make W=1 O=build_dir ARCH=i386 SHELL=/bin/bash fs/ufs/