examples/ipsec-secgw: fix GCM IV length

examples/ipsec-secgw: fix GCM IV length

The example IPsec application does not work properly when using
AES-GCM with crypto_openssl.

ESP with AES-GCM uses standard 96bit long algorithm IV ([1]) which
later concatenated with be32(1) forms a J0 block. GCM specification
([2], chapter 7.1) states that when length of IV is different than
96b, in order to format a J0 block, GHASH function must be used.

According to specification ([2], chapter 5.1.1) GCM implementations
should support standard 96bit IVs, other lengths are optional. Every
DPDK cryptodev supports 96bit IV and few of them supports 128bit
IV as well (openssl, mrvl, ccp). When passing iv::length=16 to a
cryptodev which does support standard IVs only (e.g. qat) it
implicitly uses starting 96 bits. On the other hand, openssl follows
specification and uses GHASH to compute J0 for that case which results
in different than expected J0 values used for encryption/decryption.

Fix an inability to use AES-GCM with crypto_openssl by changing IV
length to the standard value of 12.

[1] RFC4106, section "4. Nonce format" and "3.1. Initialization Vector"
    https://tools.ietf.org/html/rfc4106
[2] NIST SP800-38D
    https://csrc.nist.gov/publications/detail/sp/800-38d/final

Fixes: 0fbd75a99f ("cryptodev: move IV parameters to session")
Cc: stable@dpdk.org
Signed-off-by: Marcin Smoczynski <marcinx.smoczynski@intel.com>
Acked-by: Akhil Goyal <akhil.goyal@nxp.com>