examples/l2fwd-crypto: fix digest with AEAD algo

examples/l2fwd-crypto: fix digest with AEAD algo

When performing authentication verification (both for AEAD algorithms,
such as AES-GCM, or for authentication algorithms, such as SHA1-HMAC),
the digest address is calculated based on the packet size and the
algorithm used (substracting digest size and IP header to the packet size).

However, for AEAD algorithms, this was not calculated correctly,
since the digest size was not being substracted.

Bugzilla ID: 44
Fixes: 2661f4fbe93d ("examples/l2fwd-crypto: add AEAD parameters")
Cc: stable@dpdk.org
Reported-by: Ankur Dwivedi <ankur.dwivedi@cavium.com>
Signed-off-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>
Tested-by: Ankur Dwivedi <ankur.dwivedi@cavium.com>