scsi: target: Allow backends to hook into PR handling

For the cases where you want to export a device to a VM via a single
I_T nexus and want to passthrough the PR handling to the physical/real
device you have to use pscsi or tcmu. Both are good for specific uses
however for the case where you want good performance, and are not using
SCSI devices directly (using DM/MD RAID or multipath devices) then we are
out of luck.

The following patches allow iblock to mimimally hook into the LIO PR code
and then pass the PR handling to the physical device. Note that like with
the tcmu an pscsi cases it's only supported when you export the device via
one I_T nexus.

This patch adds the initial LIO callouts. The next patch will modify
iblock.

Signed-off-by: Mike Christie <michael.christie@oracle.com>
Link: https://lore.kernel.org/r/20230407200551.12660-16-michael.christie@oracle.com


Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>