percpu: add missing lockdep_assert_held to func pcpu_free_area

Add a missing lockdep_assert_held for pcpu_lock to improve consistency
and safety throughout mm/percpu.c.
Signed-off-by: Dennis Zhou <dennisz@fb.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

mm/percpu.c

@@ -672,6 +672,8 @@ static void pcpu_free_area(struct pcpu_chunk *chunk, int freeme,
 	int to_free = 0;
 	int *p;
 
+	lockdep_assert_held(&pcpu_lock);
+
 	freeme |= 1;	/* we are searching for <given offset, in use> pair */
 
 	i = 0;