All ->execute_task instances now need to complete the I/O explicitly,
which can either happen synchronously or asynchronously.

Note that a lot of the CDB emulations appear to return success even if
some lowlevel operations failed.  Given that this is an existing issue
this patch doesn't change that fact.

(nab: Adding missing switch breaks in PR-IN + PR_OUT)
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NNicholas Bellinger <nab@linux-iscsi.org>