scsi: iscsi: Add helper functions to manage iscsi_cls_conn

scsi: iscsi: Add helper functions to manage iscsi_cls_conn

 - iscsi_alloc_conn(): Allocate and initialize iscsi_cls_conn

 - iscsi_add_conn(): Expose iscsi_cls_conn to userspace via sysfs

 - iscsi_remove_conn(): Remove iscsi_cls_conn from sysfs

Link: https://lore.kernel.org/r/20220310015759.3296841-2-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>