rcutorture: Enable limited callback-flooding tests of SRCU

This commit allows up to 50,000 callbacks worth of callback-flooding
tests of SRCU.  The goal of this change is to exercise Tree SRCU's
ability to transition from SRCU_SIZE_SMALL to SRCU_SIZE_BIG triggered
by callback-queue-time lock contention.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>