sched/mmcid: Remove pointless preempt guard

This is a leftover from the early versions of this function where it could
be invoked without mm::mm_cid::lock held.

Remove it and add lockdep asserts instead.

Fixes: 653fda7 ("sched/mmcid: Switch over to the new mechanism")
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260310202526.116363613@kernel.org

Author: Thomas Gleixner

kernel/sched/core.c

@@ -10684,6 +10684,8 @@ static void mm_cid_fixup_tasks_to_cpus(void)
 
 static bool sched_mm_cid_add_user(struct task_struct *t, struct mm_struct *mm)
 {
+	lockdep_assert_held(&mm->mm_cid.lock);
+
 	t->mm_cid.active = 1;
 	mm->mm_cid.users++;
 	return mm_update_max_cids(mm);
@@ -10736,12 +10738,12 @@ static void sched_mm_cid_fork(struct task_struct *t)
 
 static bool sched_mm_cid_remove_user(struct task_struct *t)
 {
+	lockdep_assert_held(&t->mm->mm_cid.lock);
+
 	t->mm_cid.active = 0;
-	scoped_guard(preempt) {
-		/* Clear the transition bit */
-		t->mm_cid.cid = cid_from_transit_cid(t->mm_cid.cid);
-		mm_unset_cid_on_task(t);
-	}
+	/* Clear the transition bit */
+	t->mm_cid.cid = cid_from_transit_cid(t->mm_cid.cid);
+	mm_unset_cid_on_task(t);
 	t->mm->mm_cid.users--;
 	return mm_update_max_cids(t->mm);
 }