rcutorture: Properly set rcu_fwds for OOM handling

The conversion of rcu_fwds to dynamic allocation failed to actually
allocate the required structure.  This commit therefore allocates it,
frees it, and updates rcu_fwds accordingly.  While in the area, it
abstracts the cleanup actions into rcu_torture_fwd_prog_cleanup().

Fixes: 5155be99 ("rcutorture: Dynamically allocate rcu_fwds structure")
Reported-by: kernel test robot <rong.a.chen@intel.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>