dpif-netdev: Allow direct destroy of 'struct dp_netdev_port'.

dpif-netdev: Allow direct destroy of 'struct dp_netdev_port'.

Before this commit, when 'struct dp_netdev_port' is deleted from
'dpif-netdev' datapath, if there is pmd thread, the pmd thread
will release the last reference to the port and ovs-rcu postpone
the destroy.  However, the delayed close of object like 'struct
netdev' could cause failure in immediate re-add or reconfigure of
the same device.

To fix the above issue, this commit uses condition variable and
makes the main thread wait for pmd thread to release the reference
when deleting port.  Then, the main thread can directly destroy the
port.

Reported-by: Cian Ferriter <cian.ferriter@intel.com>
Signed-off-by: Alex Wang <alexw@nicira.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>