scsi: target: fix __transport_register_session locking

When __transport_register_session is called from transport_register_session
irqs will already have been disabled, so we do not want the unlock irq call
to enable them until the higher level has done the final
spin_unlock_irqrestore/ spin_unlock_irq.

This has __transport_register_session use the save/restore call.

Signed-off-by: Mike Christie <mchristi@redhat.com>
Reviewed-by: Bart Van Assche <bart.vanassche@wdc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>