illumos gate

Running fio with `ioengine=solarisaio` I see this:

1. dtrace -n 'io:::start,io:::done{ @[probefunc, probename] = count(); }'
   dtrace: description 'io:::start,io:::done' matched 10 probes
   ^C

bdev_strategy                                       start                                                           135
  biodone                                             done                                                            135
  aphysio                                             start                                                        174049
```

The calls through `bdev_strategy` are for the root pool and have corresponding probe firings for io:::done. The ones through `aphysio` never have a corresponding `io:::done`

It looks like this is simply due to not setting B_STARTED in bp->b_flags as we do in other paths that lead to `biodone`