torture: Make SRCU lockdep testing use srcu_read_lock_nmisafe()

Recent experience shows that the srcu_read_lock_nmisafe() and
srcu_read_unlock_nmisafe() functions are not sufficiently tested.
This commit therefore causes the torture.sh script's SRCU lockdep testing
to use these two functions.  This will cause these two functions to
be regularly tested by several developers (myself included) 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>