dpif-netdev: Fix a race.

On current master, the 'struct dp_netdev_port' is destroyed
immediately when the ref count reaches 0.  However, non-pmd
threads calling the dpif_netdev_execute() for sending packets
could hold pointer to 'port' that is not ref-counted.  Thusly
those threads could possibly access freed memory when the port
is deleted.

To fix this bug, this commit makes non-pmd threads acquiring
the 'port_mutex' before doing the actual execution in
dpif_netdev_execute().

Signed-off-by: Alex Wang <alexw@nicira.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>