reproduce (this is a W=1 build):
        # https://github.com/kdave/btrfs-devel/commit/185fa5c7ac5add0f1a109a22a972ca99780ee49e
        git remote add kdave-btrfs-devel https://github.com/kdave/btrfs-devel.git
        git fetch --no-tags kdave-btrfs-devel misc-next
        git checkout 185fa5c7ac5add0f1a109a22a972ca99780ee49e
        # 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