This permits requests to provide the AAD in a separate side buffer
instead of as a region in the crypto request input buffer. This is
useful when the main data buffer might not contain the full AAD
(e.g. for TLS or IPsec with ESN).
Unlike separate IVs which are constrained in size and stored in an
array in struct cryptop, separate AAD is provided by the caller
setting a new crp_aad pointer to the buffer. The caller must ensure
the pointer remains valid and the buffer contents static until the
request is completed (e.g. when the callback routine is invoked).
As with separate output buffers, not all drivers support this feature.
Consumers must request use of this feature via a new session flag.
To aid in driver testing, kern.crypto.cryptodev_separate_aad can be
set to force /dev/crypto requests to use a separate AAD buffer.