Fix racy assignment of zcb.zcb_haderrors

zcb_haderrors will be modified in zdb_blkptr_done, which is
asynchronous. So we must move this assignment after zio_wait.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: loli10K <ezomori.nozomu@gmail.com>
Signed-off-by: Chunwei Chen <david.chen@nutanix.com>
Closes #7099