dpif-netdev: delete lost packets in dp_execute_cb()

This commit fixes memory leaks in dp_execute_cb() in two cases:
    - when the output port cannot be found
    - when the recirculation depth is exceeded

Reported-by: Pravin Shelar <pshelar@nicira.com>
Signed-off-by: Daniele Di Proietto <ddiproietto@vmware.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>