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>

mm/percpu-stats.c

@@ -3,7 +3,7 @@
  * mm/percpu-debug.c
  *
  * Copyright (C) 2017		Facebook Inc.
- * Copyright (C) 2017		Dennis Zhou <dennisz@fb.com>
+ * Copyright (C) 2017		Dennis Zhou <dennis@kernel.org>
  *
  * Prints statistics about the percpu allocator and backing chunks.
  */

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