scsi: target: iscsi: Add upcast helpers

scsi: target: iscsi: Add upcast helpers

iSCSI target is cluttered with open-coded container_of() conversions from
se_nacl to iscsi_node_acl. The code could be cleaned by introducing a
helper - to_iscsi_nacl() (similar to other helpers in target core).

While at it, make another iSCSI conversion helper consistent and rename
iscsi_tpg() helper to to_iscsi_tpg().

Link: https://lore.kernel.org/r/20220523095905.26070-2-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>