dpif-netdev: Fix (packet) memory leaks in the slow path.

If a packet didn't match a rule in the fast path classifier its memory was
never freed. The issue was particularly clear with DPDK devices because it was
not possible to process more than ~250000 DPDK mbufs in the slow path.

This commit fixes the problem by:
* calling dpif_packet_delete() if the upcalls are disabled
* passing may_steal==true to dp_netdev_execute_actions() during normal upcall
  processing

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