rcu/nocb: Atomic ->len field in rcu_segcblist structure

Upcoming ->nocb_lock contention-reduction work requires that the
rcu_segcblist structure's ->len field be concurrently manipulated,
but only if there are no-CBs CPUs in the kernel.  This commit
therefore makes this ->len field be an atomic_long_t, but only
in CONFIG_RCU_NOCB_CPU=y kernels.

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>