cryptosoft: Don't treat CRYPTO_NULL_HMAC as an hmac algorithm.

Despite it's name, CRYPTO_NULL_HMAC is actually an unkeyed digest
similar to plain SHA-1 rather than a keyed HMAC algorithm.

Reported by: syzbot+17d7c656f1a797b0faae@syzkaller.appspotmail.com
Discussed with: markj
Fixes: 6113a08b98e4 cryptosoft: Fully support per-operation keys for auth algorithms.
Sponsored by: The FreeBSD Foundation