rcutorture: Mark rcu_torture_init as __init

The corresponding rcu_torture_cleanup cannot get marked as __exit, because
rcu_torture_init uses it to clean up if init fails.

Signed-off-by: Josh Triplett <josh@freedesktop.org>
Acked-by: "Paul E. McKenney" <paulmck@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>