Simplify swcr_authcompute() after removal of deprecated algorithms.

[jhb]

Summary

• Just use sw->octx != NULL to handle the HMAC case when finalizing the MAC.
• Explicitly zero the on-stack auth context.

Test Plan

• cryptocheck -d soft -all -z
• also tested with some KTLS changes which used HMAC with AES-CBC

[markj]

sys/opencrypto/cryptosoft.c

361 ↗

(On Diff #77916)

We are now doing some extra work for CRYPTO_NULL_HMAC, but I doubt it matters in any meaningful way.

[jhb]

sys/opencrypto/cryptosoft.c

361 ↗

(On Diff #77916)

Ah, the correct fix for that would be to not allocate a sw_octx for NULL_HMAC then. However, it's probably not worth fixing. I do think the real solution for NULL_CBC and NULL_HMAC is that IPsec should handle NULL ciphers explicitly and not (ab)use OCF. That would avoid the need for having to deal with the context switches, etc.