examples/ipsec-secgw: fix crypto-op might never get dequeued

examples/ipsec-secgw: fix crypto-op might never get dequeued

In some cases crypto-ops could never be dequeued from the crypto-device.
The easiest way to reproduce:
start ipsec-secgw with crypto-dev and send to it less then 32 packets.
none packets will be forwarded.
Reason for that is that the application does dequeue() from crypto-queues
only when new packets arrive.
This patch makes the app to call dequeue() on a regular basis.
Also to make code cleaner and easier to understand,
it separates crypto-dev enqueue() and dequeue() code paths.
pkt_process() now only enqueues packets into crypto device,
dequeuing and final processing is done by drain_crypto_queues().

Fixes: c64278c0c18b ("examples/ipsec-secgw: rework processing loop")
Cc: stable@dpdk.org
Signed-off-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Acked-by: Radu Nicolau <radu.nicolau@intel.com>
Acked-by: Akhil Goyal <akhil.goyal@nxp.com>