rcutorture: Suppressing read-exit testing is not an error

Currently, specifying the rcutorture.read_exit_burst=0 kernel boot
parameter will result in a -EINVAL exit code that will stop the rcutorture
test run before it has fully initialized.  This commit therefore uses a
zero exit code in that case, thus allowing rcutorture.read_exit_burst=0
to complete normally.

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