Arm CQ early enough in pingpong examples

Arm CQ early enough in pingpong examples

The example pingpong programs have a race when using events where the
client sends the first packet but the server hasn't yet armed the CQ
by calling ibv_req_notify_cq() thus waiting forever in
ibv_get_cq_event().  The fix is to move the call to
ibv_req_notify_cq() before signaling the client to "start".

Signed-off-by: Ralph Campbell <ralphc@pathscale.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>