mxqd: Ignore failure to scan JOB_TMPDIR_MNTDIR

Avoid unneeded warning when scanning JOB_TMPDIR_MNTDIR for possible
leftover job mounts. The parent directory of JOB_TMPDIR_MNTDIR is
created when the first job with the --tmpdir feature is started. So
it won't exist in a new server.

This cleanup path is only used in the exceptional case that the usual
cleanup path (via job_has_finished) didn't succeed, e.g. when mxqd was
killed. The cleanup is not essential for the currently running server.

mxqd.c

@@ -2308,10 +2308,8 @@ static void server_umount_stale_job_mountdirs(struct mxq_server *server) {
     int i;
 
     entries=scandir(MXQ_JOB_TMPDIR_MNTDIR,&namelist,&job_mountdirs_is_valid_name,&alphasort);
-    if (entries<0) {
-        mx_log_err("scandir %s: %m", MXQ_JOB_TMPDIR_MNTDIR);
+    if (entries<=0)
         return;
-    }
     for (i=0;i<entries;i++) {
         if (job_mountdirs_is_valid_name_parse(namelist[i]->d_name, &job_id)) {
             if (server_get_job_list_by_job_id(server, job_id) == NULL) {