NILFS_LAST_SEGBUF
prev = NILFS_LAST_SEGBUF(&sci->sc_write_logs);
prev = NILFS_LAST_SEGBUF(&sci->sc_segbufs);
bh = NILFS_LAST_SEGBUF(logs)->sb_super_root;
segbuf = NILFS_LAST_SEGBUF(&sci->sc_write_logs);
bh_sr = NILFS_LAST_SEGBUF(&sci->sc_segbufs)->sb_super_root;