btrfs: scrub: introduce the main read repair worker for scrub_stripe

The new helper, scrub_stripe_read_repair_worker(), would handle the
read-repair part:

- Wait for the previous submitted read IO to finish

- Verify the contents of the stripe

- Go through the remaining mirrors, using as large blocksize as possible
  At this stage, we just read out all the failed sectors from each
  mirror and re-verify.
  If no more failed sector, we can exit.

- Go through all mirrors again, sector-by-sector
  This time, we read sector by sector, this is to address cases where
  one bad sector mismatches the drive's internal checksum, and cause the
  whole read range to fail.

  We put this recovery method as the last resort, as sector-by-sector
  reading is slow, and reading from other mirrors may have already fixed
  the errors.

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