Fixes in head_errlog feature with encryption

For the head_errlog feature use dsl_dataset_hold_obj_flags() instead of
dsl_dataset_hold_obj() in order to enable access to the encryption keys
(if loaded). This enables reporting of errors in encrypted filesystems
which are not mounted but have their keys loaded.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: George Amanakis <gamanakis@gmail.com>
Closes #14837