rcutorture: Make scenario SRCU-P use srcu_read_lock_fast()

This commit causes the rcutorture SRCU-P scenario use the
srcu_read_lock_fast() and srcu_read_unlock_fast() functions.  This will
cause these two functions to be regularly tested by several developers
(myself included), for example, those who use torture.sh as an RCU
acceptance test.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>