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

(cherry picked from commit cb8164b05e3bad5586c2a109bbdbab1ad65a1a6f)