rcutorture: Test SRCU size transitions

Thie commit adds kernel boot parameters to the SRCU-N and SRCU-P
rcutorture scenarios to cause SRCU-N to test contention-based resizing
and SRCU-P to test init_srcu_struct()-time resizing.  Note that this
also tests never-resizing because the contention-based resizing normally
takes some minutes to make the shift.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

tools/testing/selftests/rcutorture/configs/rcu/SRCU-N.boot

 rcutorture.torture_type=srcu
+rcutorture.fwd_progress=3

tools/testing/selftests/rcutorture/configs/rcu/SRCU-P.boot

 rcutorture.torture_type=srcud
 rcupdate.rcu_self_test=1
+rcutorture.fwd_progress=3
+srcutree.big_cpu_lim=5