examples/l2fwd-crypto: fix padding

examples/l2fwd-crypto: fix padding

L2fwd-crypto app was padding an incoming buffer,
to be aligned with the algorithm block size, in all cases.
This was not the right approach, as padding is only necessary
when using block cipher algorithms, such as AES-CBC.
In case of using a stream cipher algorithm, such as SNOW3G UEA2,
there is no need to include padding and increase the buffer size.

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

Signed-off-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>
Acked-by: Fan Zhang <roy.fan.zhang@intel.com>