This patch introduces the following structures:

- scrub_sector_verification
  Contains all the needed info to verify one sector (data or metadata).

- scrub_stripe
  Contains all needed members (mostly bitmap based) to scrub one stripe
  (with a length of BTRFS_STRIPE_LEN).

The basic idea is, we keep the existing per-device scrub behavior, but
merge all the scrub_bio/scrub_bio into one generic structure, and read
the full BTRFS_STRIPE_LEN stripe on the first try.

This means we will read some sectors which are not scrub target, but
that's fine. At dev-replace time we only writeback the utilized and good
sectors, and for read-repair we only writeback the repaired sectors.

With every read submitted in BTRFS_STRIPE_LEN, the need for complex bio
form shaping would be gone.
Although to get the same performance of the old scrub behavior, we would
need to submit the initial read for two stripes at once.
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>