Better user experience for errata 4

This patch attempts to address some user concerns that have arisen
since errata 4 was introduced.

* The errata warning has been made less scary for users without
  any encrypted datasets.

* The errata warning now clears itself without a pool reimport if
  the bookmark_v2 feature is enabled and no encrypted datasets
  exist.

* It is no longer possible to create new encrypted datasets without
  enabling the bookmark_v2 feature, thus helping to ensure that the
  errata is resolved.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tom Caputi <tcaputi@datto.com>
Issue ##8308
Closes #8504

module/zfs/dsl_crypt.c

@@ -1837,6 +1837,13 @@ dmu_objset_create_crypt_check(dsl_dir_t *parentdd, dsl_crypto_params_t *dcp,
 		return (SET_ERROR(EOPNOTSUPP));
 	}
 
+	/* Check for errata #4 (encryption enabled, bookmark_v2 disabled) */
+	if (parentdd != NULL &&
+	    !spa_feature_is_enabled(parentdd->dd_pool->dp_spa,
+	    SPA_FEATURE_BOOKMARK_V2)) {
+		return (SET_ERROR(EOPNOTSUPP));
+	}
+
 	/* handle inheritance */
 	if (dcp->cp_wkey == NULL) {
 		ASSERT3P(parentdd, !=, NULL);

module/zfs/zfeature.c

@@ -376,6 +376,19 @@ feature_enable_sync(spa_t *spa, zfeature_info_t *feature, dmu_tx_t *tx)
 		    spa->spa_feat_enabled_txg_obj, feature->fi_guid,
 		    sizeof (uint64_t), 1, &enabling_txg, tx));
 	}
+
+	/*
+	 * Errata #4 is mostly a problem with encrypted datasets, but it
+	 * is also a problem where the old encryption feature did not
+	 * depend on the bookmark_v2 feature. If the pool does not have
+	 * any encrypted datasets we can resolve this issue simply by
+	 * enabling this dependency.
+	 */
+	if (spa->spa_errata == ZPOOL_ERRATA_ZOL_8308_ENCRYPTION &&
+	    spa_feature_is_enabled(spa, SPA_FEATURE_ENCRYPTION) &&
+	    !spa_feature_is_active(spa, SPA_FEATURE_ENCRYPTION) &&
+	    feature->fi_feature == SPA_FEATURE_BOOKMARK_V2)
+		spa->spa_errata = 0;
 }
 
 static void