reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/snitzer/linux.git/commit/?id=60f26fd3eccfac7de2bab5613745670132e08ae7
        git remote add snitzer https://git.kernel.org/pub/scm/linux/kernel/git/snitzer/linux.git
        git fetch --no-tags snitzer nfs-localio-for-next-nfsd_file
        git checkout 60f26fd3eccfac7de2bab5613745670132e08ae7
        # 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/nfs/flexfilelayout/