scsi: iscsi: Fix inappropriate use of put_device()

scsi: iscsi: Fix inappropriate use of put_device()

kfree(conn) is called inside put_device(&conn->dev) which could lead to
use-after-free. In addition, device_unregister() should be used here rather
than put_deviceO().

Link: https://lore.kernel.org/r/20201120074852.31658-1-miaoqinglang@huawei.com
Fixes: f159794c7ca3 ("scsi: iscsi: Fail session and connection on transport registration failure")
Reported-by: Hulk Robot <hulkci@huawei.com>
Reviewed-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Qinglang Miao <miaoqinglang@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>