k_inodes_wb
  writeback: simplify and split bdi_start_writeback
  writeback: simplify wakeup_flusher_threads
  writeback: fix writeback_inodes_wb from writeback_inodes_sb
  writeback: enforce s_umount locking in writeback_inodes_sb
  writeback: queue work on stack in writeback_inodes_sb
  writeback: fix writeback completion notifications