zdtm/tempfs_subns: sync children with the main process

All static tests has to stop any activity before C/R.

./tempfs_subns --pidfile=tempfs_subns.pid --outfile=tempfs_subns.out --dirname=tempfs_subns.test
Run criu dump
Unable to kill 128: [Errno 3] No such process
Run criu restore
7: Old mounts lost: []
7: New mounts appeared: [('/rootfs/criu/test', '/'), ('/', '/proc'), ('/', '/dev/pts')]
:
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

test/zdtm/static/tempfs_subns.c

@@ -42,26 +42,26 @@ int main(int argc, char **argv)
 
 		pid = fork();
 		if (pid == 0) {
-			if (write(fds[1], &fd, sizeof(fd)) != sizeof(fd)) {
-				pr_perror("write");
-				return 1;
-			}
 			if (unshare(CLONE_NEWNS)) {
 				pr_perror("unshare");
 				return 1;
 			}
 			prctl(PR_SET_PDEATHSIG, SIGKILL, 0, 0, 0);
+			if (write(fds[1], &fd, sizeof(fd)) != sizeof(fd)) {
+				pr_perror("write");
+				return 1;
+			}
 			while (1)
 				sleep(1);
 			return 1;
 		}
 		pid = fork();
 		if (pid == 0) {
+			prctl(PR_SET_PDEATHSIG, SIGKILL, 0, 0, 0);
 			if (write(fds[1], &fd, sizeof(fd)) != sizeof(fd)) {
 				pr_perror("write");
 				return 1;
 			}
-			prctl(PR_SET_PDEATHSIG, SIGKILL, 0, 0, 0);
 			while (1)
 				sleep(1);
 			return 1;