btrfs: raid56: make it more explicit that cache rbio should have all its data sectors uptodate

For Btrfs RAID56, we have a caching system for btrfs raid bios (rbio).

We call cache_rbio_pages() to mark a qualified rbio ready for cache.

The timing happens at:

- finish_rmw()

  At this timing, we have already read all necessary sectors, along with
  the rbio sectors, we have covered all data stripes.

- __raid_recover_end_io()

  At this timing, we have rebuild the rbio, thus all data sectors
  involved (either from stripe or bio list) are uptodate now.

Thus at the timing of cache_rbio_pages(), we should have all data
sectors uptodate.

This patch will make it explicit that all data sectors are uptodate at
cache_rbio_pages() timing, mostly to prepare for the incoming
verification at RMW time.

This patch will add:

- Extra ASSERT()s in cache_rbio_pages()
  This is to make sure all data sectors, which are not covered by bio,
  are already uptodate.

- Extra ASSERT()s in steal_rbio()
  Since only cached rbio can be stolen, thus every data sector should
  already be uptodate in the source rbio.

- Update __raid_recover_end_io() to update recovered sector->uptodate
  Previously __raid_recover_end_io() will only mark failed sectors
  uptodate if it's doing an RMW.

  But this can trigger new ASSERT()s, as for recovery case, a recovered
  failed sector will not be marked uptodate, and trigger ASSERT() in
  later cache_rbio_pages() call.

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