Add explicit _lazy_tlb annotated functions for lazy mm refcounting.
This makes lazy mm references more obvious, and allows explicit
refcounting to be removed if it is not used.

...

--- a/kernel/kthread.c
+++ b/kernel/kthread.c

@@ -1314,14 +1314,14 @@ void kthread_use_mm(struct mm_struct *mm)
 	WARN_ON_ONCE(!(tsk->flags & PF_KTHREAD));
 	WARN_ON_ONCE(tsk->mm);
 
+	mmgrab(mm);
+
 	task_lock(tsk);
 	/* Hold off tlb flush IPIs while switching mm's */
 	local_irq_disable();
 	active_mm = tsk->active_mm;
-	if (active_mm != mm) {
-		mmgrab(mm);
+	if (active_mm != mm)
 		tsk->active_mm = mm;
-	}