scsi: target: iscsi: Extract auth functions

Create functions that answers simple questions: Whether authentication is
required, what credentials, whether connection is autenticated.

Link: https://lore.kernel.org/r/20220523095905.26070-3-d.bogdanov@yadro.com


Reviewed-by: Roman Bolshakov <r.bolshakov@yadro.com>
Reviewed-by: Konstantin Shelekhin <k.shelekhin@yadro.com>
Reviewed-by: Mike Christie <michael.christie@oracle.com>
Reviewed-by: Lee Duncan <lduncan@suse.com>
Signed-off-by: Dmitry Bogdanov <d.bogdanov@yadro.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>