rcutorture: Suppress pi-lock-across read-unlock testing for Tiny SRCU

Because Tiny srcu_read_unlock() directly calls swake_up_one(), lockdep
complains when a pi lock is held across that srcu_read_unlock().
Although this is a lockdep false positive (there is no other CPU to
complete the deadlock cycle), lockdep is what it is at the moment.
This commit therefore prevents rcutorture from holding pi lock across
a Tiny srcu_read_unlock().

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