Fixes for #6639

Fixes for #6639

Several issues were uncovered by running stress tests with zfs
encryption and raw sends in particular. The issues and their
associated fixes are as follows:

* arc_read_done() has the ability to chain several requests for
  the same block of data via the arc_callback_t struct. In these
  cases, the ARC would only use the first request's dsobj from
  the bookmark to decrypt the data. This is problematic because
  the first request might be a prefetch zio which is able to
  handle the key not being loaded, while the second might use a
  different key that it is sure will work. The fix here is to
  pass the dsobj with each individual arc_callback_t so that each
  request can attempt to decrypt the data separately.

* DRR_FREE and DRR_FREEOBJECT records in a send file were not
  having their transactions properly tagged as raw during raw
  sends, which caused a panic when the dbuf code attempted to
  decrypt these blocks.

* traverse_prefetch_metadata() did not properly set
  ZIO_FLAG_SPECULATIVE when issuing prefetch IOs.

* Added a few asserts and code cleanups to ensure these issues
  are more detectable in the future.

Signed-off-by: Tom Caputi <tcaputi@datto.com>