lsns: fix failure after zombie process

(At least on kernel 3.16), stat("/proc/${pid_of_zombie}/ns/mnt") returns
-ENOENT, as a result lsns stops scanning processes prematurely.

sys-utils/lsns.c

@@ -300,7 +300,7 @@ static int read_processes(struct lsns *ls)
 
 	while (proc_next_pid(proc, &pid) == 0) {
 		rc = read_process(ls, pid);
-		if (rc && rc != -EACCES)
+		if (rc && rc != -EACCES && rc != -ENOENT)
 			break;
 		rc = 0;
 	}