zdtm: Don't forget to create /dev/ptmx in new root

Otherwise pty tests might fault.
Reported-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

test/zdtm/lib/ns.c

@@ -62,6 +62,10 @@ static int prepare_mntns()
 			fprintf(stderr, "mkdir(/dev) failed: %m\n");
 			return -1;
 		}
+		if (mknod("/dev/ptmx", 0666 | S_IFCHR, makedev(5, 2)) && errno != EEXIST) {
+			fprintf(stderr, "mknod(/dev/ptmx) failed: %m\n");
+			return -1;
+		}
 		if (mkdir("/dev/pts", 0755) && errno != EEXIST) {
 			fprintf(stderr, "mkdir(/dev/pts) failed: %m\n");
 			return -1;