md/r5cache: gracefully handle journal device errors for writeback mode

For the raid456 with writeback cache, when journal device failed during
normal operation, it is still possible to persist all data, as all
pending data is still in stripe cache. However, it is necessary to handle
journal failure gracefully.

During journal failures, the following logic handles the graceful shutdown
of journal:
1. raid5_error() marks the device as Faulty and schedules async work
   log->disable_writeback_work;
2. In disable_writeback_work (r5c_disable_writeback_async), the mddev is
   suspended, set to write through, and then resumed. mddev_suspend()
   flushes all cached stripes;
3. All cached stripes need to be flushed carefully to the RAID array.

This patch fixes issues within the process above:
1. In r5c_update_on_rdev_error() schedule disable_writeback_work for
   journal failures;
2. In r5c_disable_writeback_async(), wait for MD_SB_CHANGE_PENDING,
   since raid5_error() updates superblock.
3. In handle_stripe(), allow stripes with data in journal (s.injournal > 0)
   to make progress during log_failed;
4. In delay_towrite(), if log failed only process data in the cache (skip
   new writes in dev->towrite);
5. In __get_priority_stripe(), process loprio_list during journal device
   failures.
6. In raid5_remove_disk(), wait for all cached stripes are flushed before
   calling log_exit().

Signed-off-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Shaohua Li <shli@fb.com>