dpif-netdev: Use hmap for ports.

dpif-netdev: Use hmap for ports.

netdev objects are hard to use with RCU, because it's not possible to
split removal and reclamation.  Postponing the removal means that the
port is not removed and cannot be readded immediately.  Waiting for
reclamation means introducing a quiescent state, and that may introduce
subtle bugs, due to the RCU model we use in userspace.

This commit changes the port container from cmap to hmap.  'port_mutex'
must be held by readers and writers.  This shouldn't have performance
impact, as readers in the fast path use a thread local cache.

Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>
Tested-by: Ilya Maximets <i.maximets@samsung.com>
Acked-by: Ilya Maximets <i.maximets@samsung.com>