zdb: don't try to load key for unencrypted dataset

Previously using -K/--key on an unencrypted dataset would trip a VERIFY,
because the dataset has nowhere to load the key into.

Now, just ignore it. This makes zdb much easier to drive when there's a
mix of encrypt and non-encrypted datasets, as the key can provided for
all of them (at least, assuming the same encryption root, which is a
common enough case).

Sponsored-by: TrueNAS
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Signed-off-by: Rob Norris <rob.norris@truenas.com>
Closes #18230

cmd/zdb/zdb.c

@@ -3481,6 +3481,14 @@ open_objset(const char *path, const void *tag, objset_t **osp)
 			    path, strerror(err));
 			return (err);
 		}
+
+		/*
+		 * Only try to load the key and unlock the dataset if it is
+		 * actually encrypted; otherwise we'll just crash. Just
+		 * ignore the -K switch entirely otherwise; it's useful to be
+		 * able to provide even if it's not needed.
+		 */
+		if ((*osp)->os_encrypted) {
 			dsl_dataset_long_hold(dmu_objset_ds(*osp), tag);
 			dsl_pool_rele(dmu_objset_pool(*osp), tag);
 
@@ -3490,6 +3498,9 @@ open_objset(const char *path, const void *tag, objset_t **osp)
 			/* release it all */
 			dsl_dataset_long_rele(dmu_objset_ds(*osp), tag);
 			dsl_dataset_rele(dmu_objset_ds(*osp), tag);
+		} else {
+			dmu_objset_rele(*osp, tag);
+		}
 	}
 
 	int ds_hold_flags = key_loaded ? DS_HOLD_FLAG_DECRYPT : 0;