scsi: target: Add support for completing commands from backend context

To complete a command several drivers just drop their reference and add
it to list to be processed by a driver specific thread. So there's no
need to go from backend context to the LIO thread then to the driver's
thread. When avoiding the LIO thread, IOPS can increase from 20-30% for
workloads like:

  fio --filename=/dev/sdb  --direct=1 --rw=randrw --bs=8K \
    --ioengine=libaio --iodepth=128  --numjobs=$jobs

where increasing jobs increases the performance improvement (this is
using NVMe drives with LIO's submit_type=1 to directly submit).

Add the infrastructure so drivers and userspace can control how to
complete a command like is done for the submission path. In this commit
there is no behavior change and we continue to defer to the LIO
workqueue thread. In the subsequent commits we will allow drivers to
report what they support and allow userspace to control the behavior.

Signed-off-by: Mike Christie <michael.christie@oracle.com>
Link: https://patch.msgid.link/20260222232946.7637-2-michael.christie@oracle.com
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

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