sched/rseq: Fix concurrency ID handling of usermodehelper kthreads

sched_mm_cid_after_execve() does not expect NULL t->mm, but it may happen
if a usermodehelper kthread fails when attempting to execute a binary.

sched_mm_cid_fork() can be issued from a usermodehelper kthread, which
has t->flags PF_KTHREAD set.

Fixes: af7f588d8f73 ("sched: Introduce per-memory-map concurrency ID")
Reported-by: kernel test robot <[e-mail]>
Reported-by: Borislav Petkov <[e-mail]>
Signed-off-by: Mathieu Desnoyers <[e-mail]>
Signed-off-by: Borislav Petkov (AMD) <[e-mail]>
Link: https://lore.kernel.org/oe-lkp/[e-mail]

kernel/sched/core.c

···
 	struct mm_struct *mm = t->mm;
 	unsigned long flags;
 
-	WARN_ON_ONCE((t->flags & PF_KTHREAD) || !t->mm);
-
+	if (!mm)
+		return;
 	local_irq_save(flags);
 	t->mm_cid = mm_cid_get(mm);
 	t->mm_cid_active = 1;
···
 
 void sched_mm_cid_fork(struct task_struct *t)
 {
-	WARN_ON_ONCE((t->flags & PF_KTHREAD) || !t->mm || t->mm_cid != -1);
+	WARN_ON_ONCE(!t->mm || t->mm_cid != -1);
 	t->mm_cid_active = 1;
 }
 #endif