core: remount /sys/fs/cgroup/ read-only after we mounted all controllers

Given that glibc searches for /dev/shm by just looking for any tmpfs we
should be more careful with providing tmpfs instances arbitrary code
might end up writing to.

src/core/mount-setup.c

@@ -338,6 +338,10 @@ int mount_cgroup_controllers(char ***join_controllers) {
                 }
         }
 
+        /* Now that we mounted everything, let's make the tmpfs the
+         * cgroup file systems are mounted into read-only. */
+        mount("tmpfs", "/sys/fs/cgroup", "tmpfs", MS_REMOUNT|MS_NOSUID|MS_NOEXEC|MS_NODEV|MS_STRICTATIME|MS_RDONLY, "mode=755");
+
         return 0;
 }