scsi: target/core: Rework the SPC-2 reservation handling code

Instead of tracking the initiator that established an SPC-2 reservation,
track the session through which the SPC-2 reservation has been
established. This patch does not change any functionality.

Cc: Mike Christie <mchristi@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Nicholas Bellinger <nab@linux-iscsi.org>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>