Revert "zdb: zdb_ddt_leak_init() reads uninitialized memory..."

This reverts commit d30db519af44b905fc52b8c8ba34f6378aa03470. With
this change applied zloop.sh fails reliably with the following ASSERT.

  zio_wait(zio_claim(NULL, zcb->zcb_spa, refcnt ? 0 : spa_min_claim_txg(
    zcb->zcb_spa), bp, NULL, NULL, ZIO_FLAG_CANFAIL)) == 0 (0x2 == 0x0)
  ASSERT at cmd/zdb/zdb.c:5452:zdb_count_block()

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #14306