rcutorture: Make grace-period kthread report match RCU flavor being tested

At the end of the test and if there are rcu_torture_writer() stalls,
rcutorture invokes show_rcu_gp_kthreads() in order to dump out information
on the RCU grace-period kthread.  This makes a lot of sense when testing
vanilla RCU, but not so much for the other flavors.  This commit therefore
allows per-flavor kthread-dump functions to be specified.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
3 files changed