scsi: iscsi: fix kabi broken in struct iscsi_transport

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5HL0X


CVE: NA

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

In struct iscsi_transport, "unbind_conn" was added and the kabi
is broken. Fix it by using "tgt_dscvr" and caps flag.

Signed-off-by: Li Nan <linan122@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>
Reviewed-by: linan <linan122@huawei.com>
Reviewed-by: Jason Yan <yanaijie@huawei.com>