scsi: target: Fix truncated PR-in ReadKeys response

commit 63ce3c384db26494615e3c8972bcd419ed71f4c4 upstream.

SPC5r17 states that the contents of the ADDITIONAL LENGTH field are not
altered based on the allocation length, so always calculate and pack the
full key list length even if the list itself is truncated.

According to Maged:

  Yes it fixes the "Storage Spaces Persistent Reservation" test in the
  Windows 2016 Server Failover Cluster validation suites when having
  many connections that result in more than 8 registrations. I tested
  your patch on 4.17 with iblock.

This behaviour can be tested using the libiscsi PrinReadKeys.Truncate test.

Cc: stable@vger.kernel.org
Signed-off-by: David Disseldorp <ddiss@suse.de>
Reviewed-by: Mike Christie <mchristi@redhat.com>
Tested-by: Maged Mokhtar <mmokhtar@petasan.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>