reproduce (this is a W=1 build):
        # https://github.com/goldwynr/linux/commit/f3c4e26cd94df8423f59e3cd0caad08193e9fbb3
        git remote add goldwynr https://github.com/goldwynr/linux
        git fetch --no-tags goldwynr buffered-iomap
        git checkout f3c4e26cd94df8423f59e3cd0caad08193e9fbb3
        # 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/btrfs/