rcutorture: Add torture tests for RCU-tasks

This commit adds torture tests for RCU-tasks.  It also fixes a bug that
would segfault for an RCU flavor lacking a callback-barrier function.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
2 files changed