head_errlog: fix use-after-free

In the commit of the head_errlog feature we introduced a bug in
dsl_dataset_promote_sync(): we may dereference origin_head and hds, both
dereferencing ddpa after calling promote_sync() on ddpa.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Chunwei Chen <david.chen@nutanix.com>
Reviewed-by: Rob Norris <robn@despairlabs.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: George Amanakis <gamanakis@gmail.com>
Closes #16272
Closes #16273