GCM-AES-256 cipher suite is defined in IEEE Std 802.1AEbn-2011.
If authenticator configured as GCM-AES-256, the distributed SAK will be
256 bits indicated by the GCM-AES-256 ID in the MKA packet.
This patch will make AES Key Unwrap to 32 bytes of SAK when identify the
ID.
Signed-off-by: xiaofeis <xiaofeis@codeaurora.org>