rcu: Update MAINTAINERS entry

This commit updates Lai Jiangshan's email address because the old
laijs@cn.fujitsu.com address will expire after July 10, 2015.

Signed-off-by: Lai Jiangshan <jiangshanlai@gmail.com>
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>