reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/sandeen/linux.git/commit/?id=bf51728fb8134742e7da61ecff42fd585d05f264
        git remote add sandeen https://git.kernel.org/pub/scm/linux/kernel/git/sandeen/linux.git
        git fetch --no-tags sandeen f2fs-mount-api
        git checkout bf51728fb8134742e7da61ecff42fd585d05f264
        # 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/f2fs/