spa_sync_props: remove pool userprops by setting empty-string

People have noted there's no way to remove a pool userprop, only zero
it. Turns vdev userprops had a method, by setting empty-string. So this
makes pool userprops follow the same behaviour.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Signed-off-by: Rob Norris <[email protected]>

Author: robn

module/zfs/spa.c

@@ -9683,9 +9683,17 @@ spa_sync_props(void *arg, dmu_tx_t *tx)
 			if (nvpair_type(elem) == DATA_TYPE_STRING) {
 				ASSERT(proptype == PROP_TYPE_STRING);
 				strval = fnvpair_value_string(elem);
-				VERIFY0(zap_update(mos,
-				    spa->spa_pool_props_object, propname,
-				    1, strlen(strval) + 1, strval, tx));
+				if (strlen(strval) == 0) {
+					/* remove the property if value == "" */
+					(void) zap_remove(mos,
+					    spa->spa_pool_props_object,
+					    propname, tx);
+				} else {
+					VERIFY0(zap_update(mos,
+					    spa->spa_pool_props_object,
+					    propname, 1, strlen(strval) + 1,
+					    strval, tx));
+				}
 				spa_history_log_internal(spa, "set", tx,
 				    "%s=%s", elemname, strval);
 			} else if (nvpair_type(elem) == DATA_TYPE_UINT64) {