reproduce (this is a W=1 build):
        # https://github.com/koverstreet/bcachefs/commit/10f9cb84b36d3a4a52e122083f5be7c96a08ba75
        git remote add koverstreet-bcachefs https://github.com/koverstreet/bcachefs
        git fetch --no-tags koverstreet-bcachefs pr/739
        git checkout 10f9cb84b36d3a4a52e122083f5be7c96a08ba75
        # 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/