DAOS-15498 rebuild: reprobe in migrate_obj_iter_cb (#14458)

[jolivier23]

migrate_system_enter possibly will yield, so need to do a re-probe before delete.

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[github-actions]

Bug-tracker data:
Ticket title is 'osa/offline_drain.py:OSAOfflineDrain.test_osa_offline_drain_after_snapshot - dfs_open() of testfile.00000013 Failed'
Status is 'Resolved'
Labels: 'ci_impact,daily_test,google-cloud-daos,triaged'
Job should run at elevated priority (1)
https://daosio.atlassian.net/browse/DAOS-15498

[daosbuild1]

LGTM. No errors found by checkpatch.

[jolivier23]

@techbasset this is one that could cause some of the corruption issues

[daosbuild1]

Test stage NLT on EL 8.8 completed with status UNSTABLE. https://build.hpdd.intel.com/job/daos-stack/job/daos//view/change-requests/job/PR-14494/1/testReport/