scsi: iscsi: Add helper functions to manage iscsi_cls_conn

mainline inclusion
from mainline-v5.18-rc1
commit ad515cad
category: bugfix
bugzilla: 187381, https://gitee.com/openeuler/kernel/issues/I5LBBP
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=ad515cada7dac3cdf5e1ad77a0ed696f5f34e0ab

--------------------------------

 - 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>

Conflict: commit 1a709181c1a0 ("[Huawei] scsi: iscsi: fix kabi broken
in struct iscsi_transport") introduce some conflicts.
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>