dpif-netdev: Make datapath and flow stats atomic.

dpif-netdev: Make datapath and flow stats atomic.

A read operation from a non atomic shared value (without external
locking) can return incorrect values.  Using the atomic semantics
prevents this from happening.

However:
* No memory barriers are used.  We don't need that kind of consistency
  for statistics (we use relaxed operations).
* The updates are not atomic, just the loads and stores.  This is ok
  because there's a single writer.

Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>
Acked-by: Ethan Jackson <ethan@nicira.com>