MAINTAINERS: Add Zqiang as a RCU reviewer

I have spent about two years studying and contributing to RCU,
and sharing RCU-related knowledge within my team, if possible,
please consider me as R ;-).

Acked-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Zqiang <qiang1.zhang@intel.com>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>