scsi: target: Don't validate ignored fields in PROUT PREEMPT

The PERSISTENT RESERVE OUT command's PREEMPT service action provides two
different functions: 1. preempting persistent reservations and 2.
removing registrations. In the latter case the spec says:

  b) ignore the contents of the SCOPE field and the TYPE field;

The code currently validates the SCOPE and TYPE fields even when PREEMPT
is called to remove registrations.

This patch achieves spec compliance by validating the SCOPE and TYPE
fields only when they will actually be used.

To confirm my interpretation of the specification I tested against HPE
3PAR storage and found the TYPE field is indeed ignored in this case.

Cc: Maurizio Lombardi <mlombard@redhat.com>
Cc: Dmitry Bogdanov <d.bogdanov@yadro.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Link: https://patch.msgid.link/20260402180342.126583-1-stefanha@redhat.com
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>