reproduce (this is a W=1 build):
        # https://github.com/btrfs/linux/commit/98077f7f2180fa996710452564ebe71adc66af59
        git remote add btrfs https://github.com/btrfs/linux.git
        git fetch --no-tags btrfs pr/1157
        git checkout 98077f7f2180fa996710452564ebe71adc66af59
        # 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/btrfs/