Automatic merge from submit-queue (batch tested with PRs 46112, 46764, 46727, 46974, 46968)
iscsi storage plugin: bkpPortal should be initialized beforehand
**What this PR does / why we need it**:
This patch is a follow up patch for the PR #46239.
The bkpPortal in DetachDisk() path should be initialized before using it.
**Special notes for your reviewer**:
/cc @rootfs @childsb
**Release note**:
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NONE
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