rcutorture: Add per-version default Kconfig fragments and module parameters

Different Kconfig parameters apply to different kernel versions, as
do different rcutorture module parameters.  This commit allows the
rcutorture test scripts to adjust for different kernel versions.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Greg KH <gregkh@linuxfoundation.org>