rcutorture: Add a textbook-style trivial preemptible RCU

rcutorture: Add a textbook-style trivial preemptible RCU

This commit adds a trivial textbook implementation of preemptible RCU
to rcutorture ("torture_type=trivial-preempt"), similar in spirit to the
existing "torture_type=trivial" textbook implementation of non-preemptible
RCU.  Neither trivial RCU implementation has any value for production use,
and are intended only to keep Paul honest in his introductory writings
and presentations.

[ paulmck: Apply kernel test robot feedback. ]

Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>