btrfs: raid56: remove the old error tracking system

Since all the recovery paths have been migrated to the new error bitmap
based system, we can remove the old stripe number based system.

This cleanup involves one behavior change:

- Rebuild rbio can no longer be merged
  Previously a rebuild rbio (caused by retry after data csum mismatch)
  can be merged, if the error happens in the same stripe.

  But with the new error bitmap based solution, it's much harder to
  compare error bitmaps.

  So here we just don't merge rebuild rbio at all.
  This may introduce some performance impact at extreme corner cases,
  but we're willing to take it.

Other than that, this patch will cleanup the following members:

- rbio::faila
- rbio::failb
  They will be replaced by per-vertical stripe check, which is more
  accurate.

- rbio::error
  It will be replace by per-vertical stripe error bitmap check.

- Allow get_rbio_vertical_errors() to accept NULL pointers for
  @faila and @failb
  Some call sites only want to check if we have errors beyond the
  tolerance.
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/raid56.h

@@ -74,12 +74,6 @@ struct btrfs_raid_bio {
 	/* How many sectors there are for each stripe */
 	u8 stripe_nsectors;
 
-	/* First bad stripe, -1 means no corruption */
-	s8 faila;
-
-	/* Second bad stripe (for RAID6 use) */
-	s8 failb;
-
 	/* Stripe number that we're scrubbing  */
 	u8 scrubp;
 
@@ -93,8 +87,6 @@ struct btrfs_raid_bio {
 
 	atomic_t stripes_pending;
 
-	atomic_t error;
-
 	wait_queue_head_t io_wait;
 
 	/* Bitmap to record which horizontal stripe has data */