rcutorture: Add per-Kconfig fragment boot parameters

Some Kconfig fragments require rcutorture module parameters to
do optimal testing, for example, a configuration for SRCU would
need rcutorture.torture_type=srcu.  This commit therefore adds a
per-Kconfig-fragment boot-parameter capability.

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