btrfs: scrub: factor out parity scrub code into a helper

btrfs: scrub: factor out parity scrub code into a helper

The function scrub_raid56_parity_stripe() is handling the parity stripe
by the following steps:

- Scrub each data stripes
  And make sure everything is fine in each data stripe

- Cache the data stripe into the raid bio

- Use the cached raid bio to scrub the target parity stripe

Extract the last two steps into a new helper,
scrub_raid56_cached_parity(), as a cleanup and make the error handling
more straightforward.

With the following minor cleanups:

- Use on-stack bio structure
  The bio is always empty thus we do not need any bio vector nor the
  block device. Thus there is no need to allocate a bio, the on-stack
  one is more than enough to cut it.

- Remove the unnecessary btrfs_put_bioc() call if btrfs_map_block()
  failed
  If btrfs_map_block() is failed, @bioc_ret will not be touched thus
  there is no need to call btrfs_put_bioc() in this case.

- Use a proper out: tag to do the cleanup
  Now the error cleanup is much shorter and simpler, just
  btrfs_bio_counter_dec() and bio_uninit().

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>