Documentation/memory-barriers.txt: Add needed ACCESS_ONCE() calls to memory-barriers.txt

The Documentation/memory-barriers.txt file was written before
the need for ACCESS_ONCE() was fully appreciated.  It therefore
contains no ACCESS_ONCE() calls, which can be a problem when
people lift examples from it.  This commit therefore adds
ACCESS_ONCE() calls.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Reviewed-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: <linux-arch@vger.kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/1386799151-2219-1-git-send-email-paulmck@linux.vnet.ibm.comSigned-off-by: Ingo Molnar <mingo@kernel.org>