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)