rcutorture: Record which grace-period primitives are tested

The rcu_torture_writer() function adapts to requested testing from module
parameters as well as the function pointers in the structure referenced
by cur_ops.  However, as long as the module parameters do not conflict
with the function pointers, this adaptation is silent.  This silence can
result in confusion as to exactly what was tested, which could in turn
result in untested RCU code making its way into mainline.

This commit therefore makes rcu_torture_writer() announce exactly which
portions of RCU's API it ends up testing.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>