armv8crypto: Fix some edge cases in the AES-GCM implementation

armv8crypto: Fix some edge cases in the AES-GCM implementation

- We were only hashing up to the first 16 bytes of the AAD.
- When computing the digest during decryption, handle the case where
  len == trailer, i.e., len < AES_BLOCK_LEN, properly.

While here:

- trailer is always smaller than AES_BLOCK_LEN, so remove a pair of
  unnecessary modulus operations.
- Replace some byte-by-byte loops with memcpy() and memset() calls.
  In particular, zero the full block before copying a partial block into
  it since we do that elsewhere and it means that the memset() length is
  known at compile time.

Reviewed by:	jhb
Sponsored by:	Ampere Computing
Submitted by:	Klara, Inc.
Differential Revision:	https://reviews.freebsd.org/D28501

(cherry picked from commit 0dc7076037a87100060309f7179ef6a01f32f99e)