Make vfs_getopts() set *error to ENOENT if the option wasn't found, so that

consumers don't have to check for both error and the return value (some of
them actually don't do it).

MFC After:	1 week

sys/kern/vfs_mount.c

@@ -1798,6 +1798,7 @@ vfs_getopts(struct vfsoptlist *opts, const char *name, int *error)
 		}
 		return (opt->value);
 	}
+	*error = ENOENT;
 	return (NULL);
 }