kerndat: redirect iptables output to /dev/null

fix for commit 68a938ea ("net/iptables: check iptables command has
wait option")

else each criu log contains iptables output:

(00.020184) Found task size of 7ffffffff000
Chain INPUT (policy ACCEPT)
target     prot opt source               destination

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination
(00.029481) irmap: Searching irmap cache in work dir
Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/kerndat.c

@@ -440,12 +440,20 @@ int kerndat_loginuid(bool only_dump)
 
 static int kerndat_iptables_has_xtlocks(void)
 {
+	int fd;
 	char *argv[4] = { "sh", "-c", "iptables -w -L", NULL };
 
+	fd = open("/dev/null", O_RDWR);
+	if (fd < 0) {
+		fd = -1;
+		pr_perror("failed to open /dev/null, using log fd for xtlocks check");
+	}
+
 	kdat.has_xtlocks = 1;
-	if (cr_system(-1, -1, -1, "sh", argv, CRS_CAN_FAIL) == -1)
+	if (cr_system(fd, fd, fd, "sh", argv, CRS_CAN_FAIL) == -1)
 		kdat.has_xtlocks = 0;
 
+	close_safe(&fd);
 	return 0;
 }