examples/l2fwd-crypto: flush buffers

examples/l2fwd-crypto: flush buffers

Crypto operations are enqueued in the crypto devices
when the crypto device buffers are full (MAX_PKT_BURST),
in order to be more efficient.

The problem is that operations might be stuck in those buffers,
if they never get full, and therefore, those operations
will never be performed.

Therefore, it is necessary to have a buffer flush mechanism,
similar to the one used for flush the TX buffers, so eventually,
all packets received are ciphered and sent out.

Fixes: 387259bd6c67 ("examples/l2fwd-crypto: add sample application")

Signed-off-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>