rcutorture: NULL rcu_torture_current earlier in cleanup code

Currently, the rcu_torture_current variable remains non-NULL until after
all readers have stopped.  During this time, rcu_torture_stats_print()
will think that the test is still ongoing, which can result in confusing
dmesg output.  This commit therefore NULLs rcu_torture_current immediately
after the rcu_torture_writer() kthread has decided to stop, thus informing
rcu_torture_stats_print() much sooner.

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