rcutorture: Annotation lockless accesses to rcu_torture_current

The rcutorture global variable rcu_torture_current is accessed locklessly,
so it must use the RCU pointer load/store primitives.  This commit
therefore adds several that were missed.

This data race was reported by KCSAN.  Not appropriate for backporting due
to failure being unlikely and due to this being used only by rcutorture.

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