btrfs: raid56: extract the vertical stripe recovery code into recover_vertical()

This refactor includes the following behavior change first:

- Don't error out if only P/Q is corrupted

  The old code will directly error out if only P/Q is corrupted.
  Although it is an logical error if we go into rebuild path with
  only P/Q corrupted, there is no need to error out.

  Just skip the rebuild and return the already good data.

Then comes the following refactor which shouldn't cause behavior
changes:

- Introduce a helper to do vertical stripe recovery

  This not only reduce one indent level, but also paves the road for
  later data checksum verification in RMW cycles.

- Sort rbio->faila/b before recovery

  So we don't need to do the same swap every vertical stripe

- Replace a BUG_ON() with ASSERT()

  Or checkpatch won't let me pass.

- Mark recovered sectors uptodate after the recover loop

- Do the cleanup for pointers unconditionally

  We only need to initialize @pointers and @unmap_array to NULL, so
  we can safely free them unconditionally.

- Mark the repaired sector uptodate in recover_vertical()

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