btrfs: properly abstract the parity raid bio handling

The parity raid write/recover functionality is currently not very well
abstracted from the bio submission and completion handling in volumes.c:

 - the raid56 code directly completes the original btrfs_bio fed into
   btrfs_submit_bio instead of dispatching back to volumes.c
 - the raid56 code consumes the bioc and bio_counter references taken
   by volumes.c, which also leads to special casing of the calls from
   the scrub code into the raid56 code

To fix this up supply a bi_end_io handler that calls back into the
volumes.c machinery, which then puts the bioc, decrements the bio_counter
and completes the original bio, and updates the scrub code to also
take ownership of the bioc and bio_counter in all cases.

Reviewed-by: Nikolay Borisov <[email protected]>
Reviewed-by: Anand Jain <[email protected]>
Tested-by: Nikolay Borisov <[email protected]>
Tested-by: Johannes Thumshirn <[email protected]>
Signed-off-by: Christoph Hellwig <[email protected]>
Signed-off-by: David Sterba <[email protected]>

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