OSSL_FN: Refactor OSSL_FN_add() and OSSL_FN_sub() for truncation

OSSL_FN: Refactor OSSL_FN_add() and OSSL_FN_sub() for truncation

OSSL_FN_mul() set a path that wasn't considered for OSSL_FN_add() and
OSSL_FN_sub(); a truncated result if the result OSSL_FN isn't large
enough to contain the full result.

This is done to keep the OSSL_FN API consistent, with a (tentative)
bonus, that the function calls become more constant time accross
repeated calls with the same size for operands and result.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/29309)