bcachefs: Internal reads can now correct errors

bcachefs: Internal reads can now correct errors

Rework the read path so that BCH_READ_NODECODE reads now also self-heal
after a read error and a successful retry - prerequisite for scrub.

- __bch2_read_endio() now handles a read that's both BCH_READ_NODECODE
  and a bounce.

  Normally, we don't want a BCH_READ_NODECODE read to ever allocate a
  split bch_read_bio: we want to maintain the relationship between the
  bch_read_bio and the data_update it's embedded in.

  But correcting read errors requires allocating a split/bounce rbio
  that's embedded in a promote_op. We do still have a 1-1 relationship,
  i.e. we only allocate a single split/bounce if it's a
  BCH_READ_NODECODE, so things hopefully don't get too crazy.

- __bch2_read_extent() now is allowed to allocate the promote_op for
  rewriting after a failed read, even if it's BCH_READ_NODECODE.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>