tools/rcu: Add drgn script to dump number of RCU callbacks

This commit adds an rcu-cbs.py drgn script that computes the number of
RCU callbacks waiting to be invoked. This information can be helpful
when managing systems that are short of memory and that have software
components that make heavy use of RCU, for example, by opening and
closing files in tight loops. (But please note that there are almost
always better ways to get your job done than by opening and closing
files in tight loops.)

Reported-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>