MAINTAINERS: Add Frederic and Neeraj to their RCU files

Adding Frederic as an RCU maintainer for kernel/rcu/tree_nocb.h given his
work with offloading and de-offloading callbacks from CPUs.  Also adding
Neeraj for kernel/rcu/tasks.h given his focused work on RCU Tasks Trace.
As in I am reasonably certain that each understands the full contents
of the corresponding file.

Cc: Neeraj Upadhyay <quic_neeraju@quicinc.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Acked-by: Frederic Weisbecker <frederic@kernel.org>