memory-barriers: Retain barrier() in fold-to-zero example

The transformation in the fold-to-zero example incorrectly omits the
barrier() directive.  This commit therefore adds it back in.

Reported-by: Pranith Kumar <pranith@gatech.edu>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --cc Documentation/memory-barriers.txt
Simple merge