rcupdate: Make rcu_barrier() more paranoia-proof

rcupdate: Make rcu_barrier() more paranoia-proof

I'm not sure my original barrier function is correct. It may allow a
CPU to exit the barrier loop, with no local work to do, while RCU work
is pending on other CPUs and needing one or more quiescent periods to
flush the work through.

Although rcu_pending() may handle this, it is easiest to follow
Linux's example and simply call_rcu() a callback function on every
CPU. When the callback has executed on every CPU, we know that all
previously-queued RCU work is completed, and we can exit the barrier.

Signed-off-by: Keir Fraser <keir@xen.org>