scsi: target: add emulate_pr backstore attr to toggle PR support

The new emulate_pr backstore attribute allows for Persistent Reservation
and SCSI2 RESERVE/RELEASE support to be completely disabled. This can be
useful for scenarios such as:

- Ensuring ATS (Compare & Write) usage on recent VMware ESXi initiators.

- Allowing clustered (e.g. tcm-user) backends to block such requests,
  avoiding the multi-node reservation state propagation.

When explicitly disabled, PR and RESERVE/RELEASE requests receive Invalid
Command Operation Code response sense data.

Signed-off-by: David Disseldorp <ddiss@suse.de>
Reviewed-by: Mike Christie <mchristi@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>