Fix dtls_get_max_record_overhead()

Fix dtls_get_max_record_overhead()

We fix dtls_get_max_record_overhead() to give a better value for the max
record overhead. We can't realistically handle the compression case so we
just ignore that.

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Hugo Landau <hlandau@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/19516)