ossl: Fix handling of separate AAD buffers in ossl_aes_gcm()

Consumers may optionally provide a reference to a separate buffer
containing AAD, but ossl_aes_gcm() didn't handle this and would thus
compute an incorrect digest.

Fixes: 9a3444d91c70 ("ossl: Add a VAES-based AES-GCM implementation for amd64")
Reviewed by: jhb
MFC after: 3 days
Sponsored by: Klara, Inc.
Sponsored by: Stormshield
Differential Revision: https://reviews.freebsd.org/D42736