um: Prohibit the VM_CLONE flag in run_helper_thread()

Directly creating helper threads with VM_CLONE using clone can
compromise the thread safety of errno. Since all these helper
threads have been converted to use os_run_helper_thread(), let's
prevent using this flag in run_helper_thread().

Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
Link: https://patch.msgid.link/20250319135523.97050-5-tiwei.btw@antgroup.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

Author: btw616

arch/um/os-Linux/helper.c

@@ -122,6 +122,10 @@ int run_helper_thread(int (*proc)(void *), void *arg, unsigned int flags,
 	unsigned long stack, sp;
 	int pid, status, err;
 
+	/* To share memory space, use os_run_helper_thread() instead. */
+	if (flags & CLONE_VM)
+		return -EINVAL;
+
 	stack = alloc_stack(0, __uml_cant_sleep());
 	if (stack == 0)
 		return -ENOMEM;