percpu: update copyright emails to dennis@kernel.org

Currently there are 3 emails tied to me in the kernel tree, I'd rather
dennis@kernel.org be the only one.

Signed-off-by: Dennis Zhou <dennis@kernel.org>
diff --git a/mm/percpu.c b/mm/percpu.c
index e984408..d7e3bc6 100644
--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -6,7 +6,7 @@
  * Copyright (C) 2009		Tejun Heo <tj@kernel.org>
  *
  * Copyright (C) 2017		Facebook Inc.
- * Copyright (C) 2017		Dennis Zhou <dennisszhou@gmail.com>
+ * Copyright (C) 2017		Dennis Zhou <dennis@kernel.org>
  *
  * The percpu allocator handles both static and dynamic areas.  Percpu
  * areas are allocated in chunks which are divided into units.  There is