reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs.git/commit/?id=6d26064a464a6f8ace90a2176a17bd6f853825e0
        git remote add brauner-vfs https://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs.git
        git fetch --no-tags brauner-vfs b4/vfs-bdev-file
        git checkout 6d26064a464a6f8ace90a2176a17bd6f853825e0
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 O=build_dir ARCH=x86_64 olddefconfig
        make W=1 O=build_dir ARCH=x86_64 SHELL=/bin/bash fs/