Don't count '@' for dataset namelen if not a snapshot

This fix pool unimportable when dataset namelen is 255

Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Fix openzfs#5432

module/zfs/dsl_dataset.c

@@ -705,7 +705,11 @@ dsl_dataset_namelen(dsl_dataset_t *ds)
 	int len;
 	VERIFY0(dsl_dataset_get_snapname(ds));
 	mutex_enter(&ds->ds_lock);
-	len = dsl_dir_namelen(ds->ds_dir) + 1 + strlen(ds->ds_snapname);
+	len = strlen(ds->ds_snapname);
+	/* add '@' if ds is a snap */
+	if (len > 0)
+		len++;
+	len += dsl_dir_namelen(ds->ds_dir);
 	mutex_exit(&ds->ds_lock);
 	return (len);
 }