scsi: libiscsi: Add iscsi_cls_conn to sysfs after initialization

iscsi_create_conn() exposed iscsi_cls_conn to sysfs prior to initialization
of iscsi_conn's dd_data. When userspace tried to access an attribute such
as the connect address, a NULL pointer dereference was observed.

Do not add iscsi_cls_conn to sysfs until it has been initialized.  Remove
iscsi_create_conn() since it is no longer used.

Link: https://lore.kernel.org/r/20220310015759.3296841-3-haowenchao@huawei.com


Reviewed-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Wenchao Hao <haowenchao@huawei.com>
Signed-off-by: Wu Bo <wubo40@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>