target: cleanup pscsi request submission

Move the entirely request allocation, mapping and submission into ->do_task.
This

 a) avoids blocking the I/O submission thread unessecarily, and
 b) simplifies the code greatly

Note that the code seems to have various error handling issues, mostly
related to bidi handling in the current form.  I've added comments about
those but not tried to fix them in this commit.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>