reproduce (this is a W=1 build):
        git remote add bcachefs https://evilpiepirate.org/git/bcachefs.git
        git fetch --no-tags bcachefs bcachefs-testing
        git checkout 05b33431c0012466dd40e9949d475adfd45dfc9b
        # 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/