Place the AAD before the plaintext/ciphertext for CIOCRYPTAEAD.

Software crypto implementations don't care how the buffer is laid out,
but hardware implementations may assume that the AAD is always before
the plain/cipher text and that the hash/tag is immediately after the end
of the plain/cipher text.

In particular, this arrangement matches the layout of both IPSec packets
and TLS frames. Linux's crypto framework also assumes this layout for
AEAD requests.

Reviewed by: cem
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D11758