[PATCH v5 00/18] Use block pr_ops in LIO

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The patches in this thread allow us to use the block pr_ops with LIO's
target_core_iblock module to support cluster applications in VMs. They
were built over Linus's tree. They also apply over linux-next and
Martin's tree and Jens's trees. 
Currently, to use windows clustering or linux clustering (pacemaker +
cluster labs scsi fence agents) in VMs with LIO and vhost-scsi, you have
to use tcmu or pscsi or use a cluster aware FS/framework for the LIO pr
file. Setting up a cluster FS/framework is pain and waste when your real
backend device is already a distributed device, and pscsi and tcmu are
nice for specific use cases, but iblock gives you the best performance and
allows you to use stacked devices like dm-multipath. So these patches
allow iblock to work like pscsi/tcmu where they can pass a PR command to
the backend module. And then iblock will use the pr_ops to pass the PR
command to the real devices similar to what we do for unmap today.

The patches are separated in the following groups:
Patch 1 - 2:
- Add block layer callouts for reading reservations and rename reservation
  error code.
Patch 3 - 5:
- SCSI support for new callouts.
Patch 6:
- DM support for new callouts.
Patch 7 - 13:
- NVMe support for new callouts.
Patch 14 - 18:
- LIO support for new callouts.

This patchset has been tested with the libiscsi PGR ops and with window's
failover cluster verification test. Note that for scsi backend devices we
need this patchset:


to handle UAs. To reduce the size of this patchset that's being done
separately to make reviewing easier. And to make merging easier this
patchset and the one above do not have any conflicts so can be merged
in different trees.

- Use []/struct_size with nvme reservation structs
- Add Keith's copywrite to pr.c
- Drop else in nvme_send_pr_command
- Fix PR_EXCLUSIVE_ACCESS_ALL_REGS use in block_pr_type_from_nvme

- Pass read_keys number of keys instead of array len
- Keep the switch use when converting between block and scsi/nvme PR
types. Drop default case so compiler spits out warning if in the future
a new value is added.
- Add helper for handling
- Use void * instead of u8* for passing data buffer.
- Rename status variable to rs.
- Have caller init buffer/structs instead of nvme/scsi callouts.
- Drop blk_status to err code.

- Fix patch subject formatting.
- Fix coding style.
- Rearrange patches so helpers are added with users to avoid compilation
- Move pr type conversion to array and add nvme_pr_type.
- Add Extended Data Structure control flag enum and use in code for checks.
- Move nvme pr code to new file.
- Add more info to patch subjects about why we need to add blk_status
to pr_ops.
- Use generic SCSI passthrough error handling interface.
- Fix checkpatch --strict errors. Note that I kept the existing coding
style that it complained about because it looked like it was the preferred
style for the code and I didn't want a mix and match.

- Drop BLK_STS_NEXUS rename changes. Will do separately.
- Add NVMe support.
- Fixed bug in target_core_iblock where a variable was not initialized
mentioned by Christoph.
- Fixed sd pr_ops UA handling issue found when running libiscsi PGR tests.
- Added patches to allow pr_ops to pass up a BLK_STS so we could return
a RESERVATION_CONFLICT status when a pr_ops callout fails.

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