block: Add PR callouts for read keys and reservation

Add callouts for reading keys and reservations. This allows LIO to support
the READ_KEYS and READ_RESERVATION commands so it can export devices to
VMs for software like windows clustering.

Signed-off-by: Mike Christie <michael.christie@oracle.com>
Link: https://lore.kernel.org/r/20230407200551.12660-2-michael.christie@oracle.com


Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>