target: stop splitting commands into multiple tasks

From hch:

The high-performance backends (iblock and rd) support tasks of unlimited
size.  With that there is no reason to keep a complex infrastructure for
splitting up commands in place.  Stop doing so and only submit a single
task per data direction.  Once this is in place we can slowly remove fields
from the task that duplicate things in the command, or move other fields
into the command.

From nab:

The benefit to IBLOCK performance by removing the additional
fast-path allocation overhead + SGL mapping to se_task->task_sg[] is now
greater than transparently supporting an received CDB I/O length that
exceeds what is allowed by backend pSCSI LLD hardware max_sectors, that
was originally supported for all backend export cases.

This change may effect some users of pSCSI users on legacy hardware, but
I think most folks are now using TYPE_DISK struct scsi_device export
with IBLOCK.  The only other place where this may can issues that cannot
be resolved with IBLOCK TYPE_DISK is using TYPE_ROM, TYPE_TAPE or other
pSCSI non TYPE_DISK export with an SCSI LLDs using a smaller
max_sectors.

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