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>