osd/: add MSG_OSD_PG_(BACKFILL|BACKFILL_REMOVE|SCAN) as recovery messages

osd/: add MSG_OSD_PG_(BACKFILL|BACKFILL_REMOVE|SCAN) as recovery messages

Otherwise, these end up as PGOpItem and therefore as immediate:

class PGOpItem : public PGOpQueueable {
...
  op_scheduler_class get_scheduler_class() const final {
    auto type = op->get_req()->get_type();
    if (type == CEPH_MSG_OSD_OP ||
  type == CEPH_MSG_OSD_BACKOFF) {
      return op_scheduler_class::client;
    } else {
      return op_scheduler_class::immediate;
    }
  }
...
};

This was probably causing a bunch of extra interference with client
ops.

Signed-off-by: Samuel Just <sjust@redhat.com>