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>

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

@@ -2,3 +2,4 @@ rcutorture.torture_type=srcud
 rcupdate.rcu_self_test=1
 rcutorture.fwd_progress=3
 srcutree.big_cpu_lim=5
+rcutorture.reader_flavor=0x8