Fix tls_cbc_digest_record is slow using SHA-384 and short messages

The formula used for this is now

kVarianceBlocks = ((255 + 1 + md_size + md_block_size - 1) / md_block_size) + 1

Notice that md_block_size=64 for SHA256, which results on the
magic constant kVarianceBlocks = 6.
However, md_block_size=128 for SHA384 leading to kVarianceBlocks = 4.

CLA:trivial

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/7342)